American helicopters flying from Iraq landed inside Syria yesterday and dropped special forces who killed eight people, the Damascus government said last night, as Washington admitted it had targeted "foreign fighters."

Syria warned that it held the US "wholly responsible for this act of aggression and all its repercussions".

It described the dead as Syrian civilians, five of them members of the same family. Syrian state television reported that the attack was against a farm near Abu Kamal, five miles from the Iraqi border. Doctors in nearby al-Sukkariya said another seven people were taken to hospital with bullet wounds.

The incident threatened to unleash a new wave of anti-American feeling in Syria and across the Middle East at a time when President Bashar al-Assad, already being courted by Europe, is looking forward to improved relations with Washington after the November 4 presidential election. News of the attack led bulletins across the Arab world last night - suggesting it will have wide resonance.

Syria summoned the US charge d'affaires in Damascus to explain the incident. It also called on the Iraqi government to prevent its airspace being used in this way in future.

Eyewitness accounts said eight US soldiers landed in two helicopters and that the dead were building workers. A senior Syrian source quoted by the official Sana news agency, said four helicopters violated Syrian airspace and described the target as a "civilian building under construction".

In Washington an unnamed military official told the Associated Press the raid had targeted elements of a "foreign fighter logistics network", and that, due to Syrian inaction, the US was "taking matters into our own hands". It was the first known American attack on Syrian soil.

Intriguingly, Farhan al-Mahalawi, mayor of the nearby Iraqi border town of Qaim, told the Reuters news agency that the targeted village had been surrounded by Syrian troops.

In Israel, a security official said Israel was not involved. Last year Israel destroyed an alleged nuclear site in northern Syria. Qaim has been a significant crossing point for foreign fighters, weapons and money entering Iraq to fuel the Sunni insurgency.

Only last Thursday, the commander of US forces in western Iraq told reporters American troops were redoubling efforts to secure the Syrian border. Major General John Kelly said Iraq's western borders with Saudi Arabia and Jordan were fairly tight as a result of good policing by security forces in those countries, but that Syria was a "different story".

Thabet Salem, a political analyst, told al-Jazeera TV that the US appeared to have mistaken building workers for infiltrators. "It will raise questions as to why this is happening towards the end of the current US administration," he said.

Late last year the then US commander, General David Petraeus, praised Syria's cooperation in reducing violence in Iraq. But Syria has since refused to restart intelligence sharing with the US until Washington recognises its assistance by returning an ambassador to Damascus.

Joshua Landis, an American expert on Syria, commented last night: "The Bush administration must assume that an Obama victory will force Syria to behave nicely in order to win favour with the new administration. Thus White House analysts may assume that it can have a "freebee" - taking a bit of personal revenge on Syria without the US paying a price."

The attack comes as Syria takes another step in from the cold today when its foreign minister, Walid al-Mualim, visits London to hear praise for its newly conciliatory policies in Lebanon - and to be urged to distance itself from Iran.

In recent months Syria has established diplomatic relations with Lebanon and held several rounds of indirect talks with Israel, with Turkey acting as broker. In July, President Assad was invited to an EU summit in Paris.