Syria has protested angrily to both the US and Iraq after what it said was a US helicopter raid inside its territory that killed eight civilians.
Syria summoned US and Iraqi envoys to condemn the "aggressive act". Iraq said the area targeted was used by militants to launch cross-border attacks in Iraq.
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The US has neither confirmed nor denied the incident. It has previously accused Syria of allowing militants into Iraq.
Syria said the US helicopters attacked a farm in the Abu Kamal border area.
If confirmed, the raid would be the first known attack by US forces inside Syrian territory, says BBC diplomatic correspondent Jonathan Marcus.
A Syrian government statement said: "Syria condemns this aggression and holds the American forces responsible for this aggression and all its repercussions."
It called for an immediate investigation.
The attack drew strong criticism from Arab League Secretary General Amr Moussa, who told reporters the incident "should not pass by without clear condemnation."
Iraqi government spokesman Ali al-Dabbagh said Baghdad was "in contact with the American side about reports regarding the attack along the frontier with Syria".
But he added that the attacked area "is a theatre of insurgent activities against Iraq using Syria as a launch pad".
Neither the Pentagon nor the White House has made any official comment. A US military spokeswoman in Baghdad said it was "in the process of investigating".
Syria's press attache in London, Jihad Makdissi, told the BBC the incident was "an outrageous crime and an act of aggression".
"If [the US has] any proof of any insurgency, instead of applying the law of the jungle and penetrating, unprovoked, a sovereign country, they should come to the Syrians first and share this information," he said.
Mr Makdissi said the US government had "proved to be irrational and they have no respect for international law or human rights".
He warned that Syria would "respond accordingly in the proper way".
The government newspaper Tishrin called the act a "war crime".
Although there has been no official US response, an unnamed US military official told the Associated Press news agency that special forces had targeted al-Qaeda linked militants moving across the border.
"We are taking matters into our own hands," the US official said.
The official said there had been considerable success closing lines of entry for foreign militants but added: "The one piece of the puzzle we have not been showing success on is the nexus in Syria."
Several days ago the commander of US forces in western Iraq said they would be increasing operations to secure the border as it had become an "uncontrolled" entry point for militants.
US intelligence believes up to 90% of the foreign fighters entering Iraq come through Syria.
Syria's official Sana news agency said that "four American helicopters violated Syrian airspace around 1645 local time [1345 GMT] on Sunday".
The government said the helicopters attacked the Sukkariyeh farm near Abu Kamal, eight kilometres (five miles) from the Iraqi border.
A building under construction was hit and four children and a married couple were among the dead, it said.
Reports of the raid vary but some said at least two helicopters landed and troops disembarked to fire on the building.
"We were at the building site when four [soldiers] came down. Four [soldiers] headed towards us and another four went towards the workers who were putting in the foundation concrete," a woman identified as the wife of a security guard told Syrian television.
"They started firing at us. My little boy ran out and as I went to protect him they shot me. There were four aircraft - two of them landed while the other two remained airborne. The aircraft also kept on firing at us. They killed the workers. When I went to catch my son they shot me. He was running towards his father."
TV pictures showed a truck riddled with bullet holes and a blood-stained floor.
Our correspondent says the timing of the incident is curious, coming right at the end of the Bush administration's period of office and at a moment when many of America's European allies - like Britain and France - are trying to broaden their ties with Damascus.