The Middle East crisis deepened today as the US warned it will not 'stand by' while Iran supports rebels in Yemen. Secretary of State John Kerry said Washington would not accept foreign interference in the country in a direct criticism of Tehran's backing of Shiite Houthi fighters. It comes as a Saudi-led coalition continues to pound anti-government forces in Yemen at the start of a third week of bombing. Scroll down for video . US Secretary of State John Kerry has warned that the US will not 'stand by' while Iran supports rebels in Yemen . A member of the Saudi border guard is stationed at a look-out point on the Saudi-Yemeni border today. A Saudi-led coalition is continuing to pound anti-government forces in Yemen . Mr Kerry told PBS television: 'There have been - there are, obviously - flights coming from Iran. Every single week there are flights from Iran and we've traced it and know this. 'Iran needs to recognise that the United States is not going to stand by while the region is destabilised or while people engage in overt warfare across lines, international boundaries in other countries.' The United States has backed the Saudi-led campaign, which launched air strikes last month as the rebels advanced on Yemen's main southern city of Aden after seizing the capital Sanaa. President Abedrabbo Mansour Hadi fled Aden for Saudi Arabia during the Houthi advance and the city has since seen heavy clashes between pro and anti-government forces. Riyadh has accused Tehran, the major Shiite power, of backing the rebels in a bid to establish a pro-Iran state on its doorstep. But Iranian Foreign Minister Mohammed Javad Zarif insisted his government wanted a swift end to the fighting, which has cost more than 640 lives since March 19, according to the World Health Organization. Military force: Saudi army tanks are seen deployed near the Saudi-Yemeni border, in southwestern Saudi Arabia . Patrol: Saudi guards drive tanks and armed vehicles along the border with Yemen . Mr Kerry, fresh from world powers striking a framework agreement with Iran on its nuclear programme, said Washington was not looking for confrontation with Tehran. 'But we're not going to step away from our alliances and our friendships and the need to stand with those who feel threatened as a consequence of the choices that Iran might be making.' In another sign of growing US support for the Saudi effort, the Pentagon said it had started daily aerial refuelling for warplanes in the coalition. The first refuelling flight took place on Tuesday night with a US Air Force KC-135 Stratotanker providing fuel for a F-15 fighter jet operated by Saudi Arabia and an F-16 flown by the United Arab Emirates, spokesman Colonel Steven Warren said. Iran's supreme leader Ayatollah Ali Khamenei has warned that last week's  framework accord with world powers was no guarantee of a final nuclear deal. And President Hassan Rouhani said separately that the Islamic republic would not sign any final agreement unless 'all economic sanctions are totally lifted on the same day'. 'What has been done so far does not guarantee an agreement, nor its contents, nor even that the negotiations will continue to the end,' said Khamenei, who has the final word on all matters of state. After a week of gruelling last-ditch negotiations, Tehran and the six powers agreed on April 2 on the framework of a deal to be finalised by the end of June reining in Iran's nuclear programme in return for the lifting of international sanctions. 'Everything is in the detail, it may be that the other side, which is unfair, wants to limit our country in the details,' Khamenei said, in his first comments on the deal. Iran's supreme leader Ayatollah Ali Khamenei has warned that last week's framework accord with world powers was no guarantee of a final nuclear deal . Playing down expectations of a deal after the interim accord - which sparked celebrations in the streets of Iranian cities - Khamenei said he had not taken any position until now as 'there is nothing to take a stance on'. 'Officials say that nothing has been done yet and there is nothing binding. I am neither for nor against.' Under the outline text agreed in the Swiss city of Lausanne between Tehran and the so-called P5+1 powers - the United States, Britain, China, France and Russia plus Germany - Iran must significantly reduce its number of centrifuges in exchange for a suspension of sanctions. The outline was a major breakthrough in a 12-year international crisis over Iran's nuclear programme. 'I have always supported and still support the Iranian negotiating team,' Khamenei said. 'I welcome any agreement that protects the interests and greatness of the nation, but having no agreement is more honourable than an agreement in which the interests and greatness of the nation is damaged.' Iranian President Hassan Rouhani said the Islamic republic would not sign any final agreement unless 'all economic sanctions are totally lifted on the same day' He said that retaining a civil nuclear industry in any agreement was vital for Iran's future development. 'The nuclear industry is a necessity, for energy production, for desalination, and in the fields of medicine, agriculture and other sectors,' he said. In a potential obstacle to any final deal, Rouhani said his country wanted sanctions lifted on the day of the implementation of any agreement. 'We will not sign any agreements unless on the first day of the implementation of the deal all economic sanctions are totally lifted on the same day,' he said. The pace at which the sanctions will be lifted is one of the outstanding issues that still has to be agreed in the final accord. Western governments, which have imposed their own sanctions over and above those adopted by the United Nations, have been pushing for it to happen only gradually. 'In return for Iran's future cooperation, we and our international partners will provide relief in phases from the sanctions that have impacted Iran's economy,' US Secretary of State John Kerry said last week. Rouhani, who was speaking on Iran's National Nuclear Technology Day, reiterated that his government remained determined to develop its civil nuclear programme. 'We will have a tanker sortie every day,' Warren said, adding that all flights will be outside of Yemeni air space. The air strikes killed at least 14 rebel fighters in Aden overnight Thursday at positions near the northern edge of the city, a source in pro-government forces told AFP. Air strikes also hit a military camp in the southern Shabwa province that was seized by the Houthis' main allies - security forces who have remained loyal to former strongman Ali Abdullah Saleh, a local official said. The official had no information on casualties among the forces loyal to Saleh, who has been accused of joining with the Houthis after being ousted from power in 2012 after an Arab Spring-inspired uprising. Yemen has been wracked by conflict since Saleh's ouster, with Hadi unable to assert government authority in a deeply tribal country riven by divisions. Supporters of the Shiite Houthi militia brandish their weapons in the Yemeni capital Sanaa. Washington has said it will not accept foreign interference in the country . Conflict: Yemeni supporters of the southern seperatist movement fire towards Houthi rebels during clashes in the southern city of Aden yesterday . As well as the Houthis, who hail from Yemen's north, the government has struggled against Al-Qaeda in the Arabian Peninsula (AQAP), considered the most dangerous of the jihadist network's branches. Authorities in Yemen had for years allowed Washington to carry out a drone war against AQAP but US forces pulled out of the country amid the latest unrest. Al-Qaeda has taken advantage of the chaos to seize control of some areas and carried out a series of deadly attacks on both government forces and the Houthis. As the fighting and air campaign drag on, concern has been growing for what aid workers say is a mounting humanitarian crisis. Some aid trickled in to Aden by ship on Wednesday but efforts by the International Committee of the Red Cross to organise flights of cargo planes into Sanaa have so far failed. In Aden, witnesses have said the situation is dire, with bodies lying in the streets and mosques calling through loudspeakers for help. Smoke and flames rise from Shiite Houthi rebel camps following an airstrike by the Saudi-led allianceearlier this week . A Yemeni child receives treatment at the burns unit of a hospital in Sanaa, following a reported airstrike by the Saudi-led coalition earlier this month . Diplomatic efforts have stepped up to resolve the conflict, with the Iranian and Pakistani foreign ministers pledging to work to find a negotiated solution. Zarif laid out a four-stage plan for talks, calling for an immediate ceasefire followed by humanitarian assistance, dialogue among Yemenis and the formation of an 'all-inclusive government'. Turkish President Recep Tayyip Erdogan was also quoted on Thursday as calling for a regional effort to end the fighting. 'The groups in Yemen should meet and work on possible solution. Saudi Arabia, Turkey and Iran should be involved in efforts for a diplomatic solution,' Erdogan was quoted as saying by Turkish media.
Secretary of State John Kerry hits out at Iran's support of Houthi fighters . But adds that Washington is not looking for a confrontation with Tehran . Saudi-led coalition starts third week of air-strikes against rebels in Yemen . Pentagon has started daily aerial refuelling for warplanes in the coalition .