The Wales Air Ambulance and three road ambulances attended the crash at Pandy on the main road between Abergavenny and Hereford at about 15:10 BST.
The A465 Hereford Road has been shut in both directions but it was expected to reopen on Thursday evening.

The new deal with French oil and gas company Total will begin with the 2017 Africa cup of Nations finals in Gabon.
For the time being the value of the deal has not been revealed.
"The parties have agreed not to release the amount of the deal for time being," Caf's director of communication, Junior Binyam, told BBC Sport.
Total take over from mobile phone company Orange, whose deal came to an end in 2016, as headline sponsor of its 10 competitions.
"This partnership is a major milestone in our ongoing search for additional resources to accelerate African football's development, bring its governance up to date, upgrade its sports infrastructure and advance its performance globally," Caf president Issa Hayatou said in a statement.
Patrick Pouyanné, president and chief executive officer of Total, had this to say on the deal: "Football is equal parts enthusiasm, sharing and team performance, concepts that resonate across cultures.
"We are delighted to partner with Caf, because Africa is part of Total's makeup.
"Through this commitment, we hope to strengthen ties to our stakeholders and customers through exciting, celebratory events that are always popular, including within our own teams."

Brown accused referee Dean Whitestone of a number of "really poor" decisions.
Whitestone awarded Gillingham a penalty in first-half stoppage time before sending off Southend's Ryan Inniss for two bookable offences.
"There were four or five decisions that says that referee shouldn't be refereeing at this level," said Brown.
The 57-year-old claimed Whitestone did not cope with the pressure of the game between two sides separated by the mouth of the Thames.
"The referee mentioned to me that it was a derby game and he'd heard about the potential friction - but you can't take that into consideration," said Brown.
"He initially crossed his arms about to wave away the penalty but the next movement he blows for a foul. That, for me, is a referee under pressure.
"The sending off is even worse - unbelievable refereeing. This was officiating at its worst and someone has to be brought to task. I'm appealing the red card and if I can take it further than that I will."
Meanwhile, Southend defender and former captain Adam Barrett has announced his retirement at the age of 37.
Barrett played 354 games in two spells with the club, the last against Stevenage in the EFL Trophy in November.
He was a member of the Shrimpers side that won back-to-back promotions from League Two to the Championship in 2005 and 2006, and also the League Two play-off final in 2015.
"He will be remembered as one of Blues' greatest and a true legend," Brown told the club website.
Two people have been airlifted to hospital after a four-vehicle crash on the A465 in Monmouthshire.
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The Confederation of African Football (Caf) has signed a new main sponsor for its top club and national team competitions for the next eight years.
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Southend manager Phil Brown says the referee for his side's 2-1 defeat at Gillingham should be "brought to task" over his performance.