The US on Monday claimed it had killed a senior agent involved in smuggling foreign fighters from Syria into Iraq when US forces carried out a raid into Syrian territory at the weekend.

A US official said in Washington that the agent, identified as Abu Ghadiya, was “one of the most prominent foreign fighter facilitators in the region”.

However, the US incursion into Syrian territory drew a strong attack from Walid al-Moualem, Syria’s foreign minister, who condemned it as an act of “criminal and terrorist aggression’’.

In London, where he held talks with David Miliband, Britain’s foreign minister, Mr Moualem demanded that Washington explain its actions which, he said, resulted in the killing of eight civilians.

“We put the responsibility on the American government,” he said, adding that he wanted the US “to investigate and return back to us with the result and explanation why they did it”.

Mr Moualem made clear in a news conference that the US action would not undermine attempts by France and the UK in recent months to establish closer relations with Damascus.

“Our relation with France is progressing and our relation with Britain will hopefully see very real progress in the near future,” he said, adding that he had invited Mr Miliband to visit Damascus in the near future.

British officials also said that Mr Moualem’s visit to London – the most high profile encounter between the two governments since 2002 – had been positive, despite the US action. “There was no suggestion in the three hours that we spent with [Mr] Moualem that the US action would have implications for the UK-Syria relationship,” the official said.

US officials said last week that US and Iraqi action had reduced the flow of insurgents from Syria into Iraq. However, the US and UK remain concerned about the issue, including the ability of foreign fighters to live relatively freely on the Syrian side of the border with Iraq.