KABUL - Eight US troops were killed by a roadside bomb in southern Afghanistan on Thursday in the deadliest single attack on foreign forces in a month, the US military said.

Afghan violence has surged in recent weeks as Taliban-led insurgents ramped up their long-expected "spring offensive."

US commanders had warned a surge in violence was likely, with militants hitting back after NATO-led forces claimed parts of the insurgency's southern stronghold over the last year.

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Thursday's bomb was the worst individual attack on foreign troops since eight US service personnel and a US contractor were shot dead by an Afghan air force pilot at a military airport in Kabul on April 27.

The Pentagon and NATO-led International Security Assistance Force (ISAF) in Afghanistan initially said seven troops were killed on Thursday, but later added that an eighth died.

Separately, another ISAF service member was killed earlier on Thursday when a helicopter crashed in eastern Afghanistan, the coalition said. The cause of that crash was under investigation.

The nearly decade-old war in Afghanistan is increasingly unpopular in the United States.

Of the roughly 2,480 foreign troops killed in Afghanistan since 2001, more than 1,580 have been US nationals.

News of the latest killings came as lawmakers in Congress narrowly lost a vote that would have required US President Barack Obama to start planning for an accelerated withdrawal.

Foreign troops are preparing to start a gradual reduction in forces from July, handing over lead security responsibility to Afghan forces by the end of 2014.

But critics of Obama's war strategy in Congress are calling for a faster drawdown, particularly in the wake of the killing of al Qaeda leader Osama bin Laden in neighboring Pakistan.

Civilian and military casualties reached record levels in 2010, the worst year of the war since US-backed Afghan forces toppled the Taliban in 2001.

A total of 711 foreign troops were killed last year and 2011 is expected to follow a similar pattern, with casualty tolls rising during the spring and summer.

Almost 200 foreign troops have been killed in Afghanistan so far in 2011.