Senior US officials claimed last night that the head of a Syrian network responsible for smuggling foreign fighters, weapons and cash into Iraq had been killed in Syria during a raid by US special forces that sparked strong condemnation from Damascus.

The Syrian foreign minister, Walid al-Moualem said the raid had killed eight civilians and was an act of "criminal and terrorist aggression." Speaking at a news conference in London, he warned that Damascus would defend itself against any such future attack.

Sunday's raid, 10km from the Iraqi border, took place in daylight and therefore was "not a mistake," he said.

The rare attack into Syria marks an unexpected expansion of the war in Iraq and comes as the level of fighting drops to its lowest level for four years.