BAGHDAD , Iraq -LRB- CNN -RRB- -- More than half the Christian population has fled the northern Iraqi city of Mosul in reaction to a campaign of killings and intimidation , according to the United Nations . Christian women attend a religious service Sunday at a church in Baghdad . An estimated 13,000 Christians have left because of oppression the U.S. government blames on al Qaeda in Iraq . Mosul , in Nineveh province , is one of the last Iraqi cities where al Qaeda in Iraq has a significant presence and routinely carries out attacks . An estimated 2,300 families have fled Mosul this month , but the exodus has started to ebb , Nineveh province 's Deputy Gov. Khasro Goran said Tuesday . No Christian family has left the city over the past seven days , and at least 100 families returned to their homes in and around Mosul during that time , Goran said . Authorities said they believe Christian demonstrations earlier this month may have prompted the attacks . Hundreds of Christians took to the streets in Mosul and surrounding villages and towns demanding adequate representation on provincial councils , whose members will be chosen in local elections in January . The violence that followed those protests left at least 14 Iraqi Christians dead and prompted the Iraqi government to dispatch more security forces to patrol the city . Violence has declined as a result , Iraqi officials said . Last week , U.S. Defense Department spokesman Geoff Morrell said the anti-Christian attacks and threats are partly `` due to elements of al Qaeda that still enjoy some ability to operate up there . '' `` This is an attempt , it appears , to try to inflame tensions and fault lines that exist between religious and sectarian groups , '' Morrell said . The U.N. High Commissioner for Refugees is helping many of the displaced families , most of whom have fled to nearby villages in Nineveh province . About 400 others have crossed into Syria , but many have said they no longer feel safe there , according to the refugee agency . Syria already hosts more than 1 million Iraqi refugees . The U.N. agency said many of the Iraqi Christians told stories of intimidation and death threats . `` One woman said she and her mother left Mosul early last week , two days after someone called one of her colleagues at work and said that all Christians should leave the city immediately or be killed , '' according to a UNHCR news release . `` She said she was unnerved but decided to leave only after hearing reports that 11 people had been killed at a checkpoint by militiamen dressed as police officers . She and her mother escaped with a couple of bags and all the money that they had in the house -- they did not dare go to the bank to remove their savings . '' A nurse told UNHCR that the threats against Christians in Mosul began months ago `` with phone calls , letters and messages left on doors . '' In other violence Tuesday in Mosul , four police officers were killed and four wounded when gunmen opened fire on their vehicle , an Iraqi Interior Ministry official said . Gunmen in cars ambushed police as they were on their way to start work in west Mosul 's al-Amil neighborhood , the official said . Also Tuesday in Baghdad , six civilians died and 23 were hurt in four bombings , the official said . In eastern Baghdad , in the Shiite al-Baladiyat neighborhood , a roadside bomb exploded Tuesday afternoon , killing one civilian and wounding five others , the official said . Earlier , a parked car exploded near a busy outdoor market in the western al-Jihad neighborhood , killing at least five people and wounding 11 others , the Interior Ministry official said . No other details were available about the blast . Two roadside bombs exploded in central Baghdad 's al-Nahdha commercial area , wounding seven people , the official said . The first blast hurt four civilians , while the second wounded three , the official said . Also Tuesday , the Iraqi army and police discovered 20 decomposed bodies in the basement of an abandoned house in Tal Afar , a town about 43 miles -LRB- 70 kilometers -RRB- west of Mosul . There was no indication of how the victims -- who could not immediately be identified -- were killed , the official said . Authorities discovered the bodies after receiving tips from local residents . CNN 's Mohammed Tawfeeq contributed to this report .