(Reuters) -- Al-Qaida has claimed responsibility for killing 48 Syrian soldiers and state employees in Iraq last week, saying their presence proved collusion between the Shiite-led government in Baghdad and Syrian President Bashar Assad.

Unidentified gunmen last week attacked a convoy of Syrians who had fled across the border into Iraq from a Syrian rebel advance, and were being escorted back home through the western province of Anbar, Iraq's Sunni Muslim heartland.

"Military detachments succeeded in annihilating an entire column of the Safavid army," al-Qaida's Iraqi wing, Islamic State of Iraq, said in a statement posted online, referring to the dynasty that ruled Shiite Iran from the 16th to 18th centuries. Tehran is Assad's closest regional ally.

"The lions of the desert and the men entrusted with difficult missions laid ambushes on the road leading to the crossing," it said.

The group said the presence of the Syrians in Iraq showed the Baghdad government's "firm co-operation" with Assad. The Syrian leader's Alawite faith is an offshoot of Shiite Islam.

Iraq's Defense Ministry has blamed the attack, which also killed nine Iraqi soldiers, on Syrian armed groups it said had infiltrated the country.

The conflict war in Syria, where mainly Sunni rebels are fighting to topple Assad, is straining Iraq's own precarious sectarian and ethnic balance of Sunnis, Shiites and Kurds.

Since December, tens of thousands of Sunni protesters have staged demonstrations, especially in Anbar province, venting frustrations that have built up since the US-led invasion of 2003 toppled Saddam Hussein and empowered majority Shiites.

Islamic State of Iraq and other Sunni Islamist groups oppose Shiite Prime Minister Nuri al-Maliki, who is close to Iran. Iraq says it takes no side in the Syrian conflict.

The militant group said the Iraqi government had failed to conceal "the reality of its firm co-operation" with Assad.

While violence in Iraq has eased since sectarian slaughter that killed tens of thousands peaked in 2006-2007, insurgents have carried out at least one major attack a month since US forces left in December 2011.

Bombings and killings still happen daily, often aimed at Shiite areas and local security forces.