The killing of 48 Syrian soldiers in Iraq threatens to entangle Baghdad in neighbouring Syria's civil war, a conflict in which it has sought to remain neutral.
A convoy of wounded Syrian soldiers who had been treated in Iraq was ambushed in the western Anbar province on Monday en route to the Syrian border, where they were to be returned, the Iraqi defence ministry said.
Nine Iraqi guards were also killed in the ambush.
Syrian President Bashar al-Assad's regime is dominated by his minority Alawite community, an offshoot of Shiite Islam, while rebels fighting to overthrow him are mainly from Syria's Sunni majority.
Iraq's government is led by the country's Shiite majority, with the country having a sizeable Sunni Arab community. The country was racked by Sunni-Shiite sectarian violence in past years that left tens of thousands dead.
Baghdad has consistently avoided joining calls for the departure of Assad, saying it opposes arming either side and urging an end to violence on all sides.
The ambush is just the latest encroachment into Iraq of the bloody and protracted conflict to jeopardise its efforts to remain outside the fray.
Political analyst Hamid Fadhel said the Syrian civil war "is a conflict with regional dimensions," which particularly threatens religiously and ethnically mixed countries, such as Iraq and Lebanon.
Monday's ambush "will increase the danger of the conflict in Syria today, and is a clear message for all Iraqis that what is happening in Syria" has moved to Iraq, Fadhel said.
John Drake, an Iraq specialist with risk consulting firm AKE Group, said the ambush potentially marked a major escalation in the spillover of the conflict.
"If this was actually by Syrian rebels, it would be the biggest incursion into Iraqi territory since the start of the fighting in Syria" nearly two years ago, he said, adding there might have been at least "some support" from Iraqis.
"The fact that the (Syrian) victims entered Iraq for their safety could prompt the Syrian rebels to view Iraq and Iraqi interests as a potential threat to their effort," he said.
"This could therefore lead to a rise in intent amongst some of the more radical anti-Assad groups to attack the Iraqi state."
Iraq is caught between conflicting pressures over Syria. Its powerful eastern neighbour, predominantly Shiite Iran, backs Assad. The United States and many Arab states want to see the president step down.
Political analyst Ihsan al-Shammari said the conflict had the potential to inflame sectarian tensions inside Iraq, saying that "if Iraq gets involved in the Syria conflict, it will be the beginning of a major armed sectarian explosion."
Official reaction to the ambush highlighted Iraq's fears of a spillover of the violence.
The defence ministry said a Syrian "terrorist group" carried out the ambush, which it termed "an attack against the sovereignty of Iraq, its land and its dignity."
Prime Minister Nuri al-Maliki's spokesman, Ali Mussawi, said: "This confirms our fears of the attempt of some to move the conflict to Iraq, but we will face these attempts by all sides with all of our power."
And parliament speaker Osama al-Nujaifi said "the Iraqi army should stay away from interfering in the Syrian matter."
It is not the first time the conflict has crossed the border.
Fire from the Syrian side killed an Iraqi soldier in northern Iraq on Saturday and a young girl in western Iraq in September.
US officials have also repeatedly called on Iraq to stop allowing overflights by Iranian planes that Washington says are being used to transport weapons to Assad's forces.
On Sunday, the Syrian National Council, a key opposition group, alleged that Iraq "gave political and intelligence support to the Syrian regime."
And like other countries bordering Syria, Iraq has seen the arrival of a flood of refugees from the conflict -- more than 105,000, according to the United Nations.