The United States on Wednesday welcomed renewed efforts to relocate residents of a camp in Iraq that for years has housed members of an Iranian opposition group.

"The United States welcomes today's safe arrival of the sixth convoy of approximately 400 Ashraf residents to Camp Hurriya, the first such convoy in over three months," State Department spokeswoman Victoria Nuland said in a statement.

"We call on the Camp Ashraf leadership to continue this progress by cooperating with the expeditious relocation of the approximately 800 remaining residents at Camp Ashraf," she added.

The Iraqi government has been locked in a dispute with the People's Mujahedeen of Iran (MEK) over plans to relocate them to Camp Hurriya, an ex-US military base formerly known as Camp Liberty, where 2,000 of the refugees have been transferred already.

Camp Ashraf is located 80 kilometers northeast of Baghdad. Under a December 2011 agreement between the United Nations and the Iraqi government, Baghdad was due to transfer the refugees from Ashraf to Hurriya. However, the relocation of 1,200 others stalled in early May.

"The government of Iraq has made considerable efforts to achieve a peaceful and secure resolution for the residents of Camp Ashraf, and we urge continued steps to address humanitarian concerns raised at Camp Hurriya by the residents," Nuland said.

The MEK was founded in the 1960s to oppose the US-backed Shah of Iran but took up arms against the country's new clerical rulers after the Islamic revolution of 1979.

The group, which has been on the US terror blacklist since 1997, says it has renounced violence and has asked Washington to remove it from the list.

The United States has asked the MEK to leave Ashraf for Hurriya and then Iraq altogether before it can be removed from the terror blacklist.

Former Iraqi president Saddam Hussein welcomed the exiles to Iraq during the 1980-88 Iraq-Iran war and they have lived at the camp ever since.

But it became a mounting issue for Iraqi authorities after US forces handed over security for the camp in January 2009, amid pressure from Tehran to hand over members of the militant group.

In April 2011, an attack by Iraqi forces on Camp Ashraf claimed 36 lives and injured 300, according to the National Council of Resistance.