Five Syrians, who were fleeing Iraq in a bus, were killed on Monday when they were hit by an American missile.
At the Pentagon, Major General Stanley McChrystal said the intended target was a bridge 160km from the Iraqi-Syrian border, and the bus "came into the pilot's view too late to recall the bomb..."
"Unintended casualties like this are regrettable," McChrystal said. "We extend our sympathies to the families of those civilians who were accidentally killed."
The bus had 37 passengers, Syrian officials said.
At least 15 passengers were reported hurt.
In Damascus, the Syrian foreign ministry summoned the US and British ambassadors to lodge an official protest over the incident, Syrian news agency SANA said.
The ministry "reserved the right to claim damages and warned against the danger of targeting innocent civilians", it said.
"This is a pure act of savagery, an outrageous and unjustifiable attack on innocent civilians," CNN quoted a Syrian government official as saying.
