Syria's foreign minister has accused the US of an act of "criminal and terrorist aggression" over what it says was a helicopter raid on its territory.
Walid Muallem said Sunday's attack saw four US aircraft travel eight miles inside Syrian airspace from Iraq and kill eight unarmed civilians on a farm.
He said those who died were a father and his three children, a farm guard and his wife, and a fisherman.
The US has not confirmed or denied the alleged raid.
However, a unnamed US official was quoted by the AFP news agency as saying that its forces had mounted a "successful" raid against foreign fighters threatening US forces in Iraq.
The US has previously accused Syria of allowing militants into Iraq, but Mr Muallem insisted his country was trying to tighten border controls.