Al-Qaida's Iraq branch claimed responsibility Monday for the deaths of 51 Syrian soldiers and nine Iraqis killed in a well-planned assault in western Iraq last week, intensifying concerns that the terror group is coordinating with Islamist rebels fighting to topple Syrian President Bashar Assad.
Iraqi authorities say fighters and weapons are moving increasingly more freely across the long and porous desert border between the two countries as Syrian rebels try to consolidate control on their side of the frontier.
The issue also plays into the conflict between Iraq's Shiite-led government and Sunni insurgents, particularly al-Qaida.
Iraq officially has not taken sides in the Syria civil war, though Iraqi Prime Minister Nouri al-Maliki warned in an interview with The Associated Press this month that a victory for rebels would create a new extremist haven and spark sectarian wars in his own country and in Lebanon.
Al-Qaida in Iraq and Syria's Jabhat al-Nusra ultimately aim to create a border sanctuary they can both exploit that could house command centers and training camps, according to two Iraqi military intelligence officials.
They estimate there are about 750 Jabhat al-Nusra militants — including foreign fighters from other Arab countries — among approximately 2,000 anti-Assad fighters who control long stretches of borderlands on the Syrian side. The officials said the Syrian militants are increasingly crossing into Iraq to meet their al-Qaida counterparts.
The Iraqi officials spoke on condition of anonymity because they were not authorized to disclose security operations to reporters.
When asked about coordination between the two groups, Ali al-Moussawi, a spokesman for Iraq's prime minister, suggested there is no difference between al-Qaida in Iraq and Syria's Jabhat al-Nusra.
"We are very concerned with what is going on the Syrian side of the border. Whatever the names, al-Qaida is one organization and in Iraq this organization has been working to ignite sectarian strife by attacking both the Sunnis and the Shiites," he said.
Iraqi officials say rebels now control the Syrian side of two desert border crossings with Iraq — at al-Qaim and Rabiya. Al-Moussawi called that a great source of concern.
The Syrian troops killed on Iraqi soil March 4 had sought refuge in northern Iraq during recent clashes that ended with the rebels taking over the Rabiya border crossing along Iraq's northern province of Ninevah. The troops were being escorted back to Syria through another border crossing further south when they were ambushed.
It was the first time Syrian soldiers were known to be in Iraq since Syria's civil war began.
Joseph Holliday, a researcher at the Institute for the Study of War, a Washington think tank, described the attack as the clearest example yet of spillover from the Syrian conflict into Iraq.
"The ambush is indicative of broader cross-border cooperation between Sunni militant groups seeking to disrupt Assad regime security forces on both sides of the border," he said in a recent report.
Iraqi officials say they allowed the Syrians in on humanitarian grounds.
In a statement posted on militant websites, the Islamic State of Iraq — al-Qaida's wing in Iraq — said its fighters were monitoring the movements of the soldiers as Iraqi authorities worked to transfer them secretly back across the border.
"We prepared for this raid after the blessed operations carried out by our brothers in Syria," the statement read, linking its cause directly to the rebels fighting to overthrow Assad's regime.
The attack started with militants detonating explosive charges on military escort vehicles assigned to protect trucks carrying the Syrian soldiers, the group said. After that, "the fighters launched an attack from two directions using light- and medium-range weapons as well as rocket-propelled grenades," said al-Qaida in Iraq.
"Within less than half an hour, the whole convoy ... was annihilated," the group said.
The account of the attack matches descriptions that Iraqi officials provided to the AP in the immediate aftermath of the assault.
Iraqi officials have launched a manhunt for the attackers, but no arrests have been made.
The Syrian conflict's sectarian divisions run deep, with predominantly Sunni rebels fighting a regime dominated by Alawites, an offshoot of Shiite Islam.
Rebel groups have increasingly embraced radical Islamic ideologies, and some of their greatest battlefield successes have been carried out by Jabhat al-Nusra.
Al-Qaida leader Ayman al-Zawahri last year urged Iraqi insurgents to support the fight to topple Syria's Assad, whose Alawite sect is a branch of Shiite Islam. Al-Qaida in Iraq considers Shiites to be heretics.
Assad's regime is backed by Shiite powerhouse Iran, which has been building ties with the Shiite-led government in Baghdad in recent years.
American officials have expressed concern about regional spillover.
The spokesman for the U.S. Embassy in Baghdad, Frank Finver, condemned last week's attack and said officials were aware of al-Qaida's claim of responsibility. He said it is "a reminder of the formidable challenges Iraq continues to face regarding its security."
"The U.S. and Iraq have long acknowledged that defeating (al-Qaida in Iraq) requires a sustained effort. We remain firmly committed to supporting the (Iraqi government's) efforts to bring greater stability to its people," Finver said.
Iraq's government, meanwhile, is being challenged by weekly protests that began in December from Sunni Muslims angry over perceived discrimination.
The demonstrations have been largely peaceful, and most Iraqi Sunnis do not voice support for al-Qaida. Many in their ranks also have concerns about Syria's violence spilling across the border.
Mahmoud al-Sumaidaie, the deputy head of Iraq's Sunni Endowment, which oversees the sect's holy sites, described last week's raid as worrying.
"There is a long border and there are tribal connections and shared beliefs between Iraq and Syria," he said in an interview Monday. "While we feel pain for what is going on in Syria, we hope that this does not spread to Iraq. ... We hope there will be no Iraqi interference in Syria's affairs, and no Syrian interference in Iraq's affairs."
Associated Press writer Sinan Salaheddin in Baghdad and Ben Hubbard in Beirut contributed.