Attacks soared by 59 per cent to 5,222 incidents from January to May, compared with 3,283 attacks in the first five months of 2008, according to US military officials and excerpts of a report by Nato's International Security Assistance Force (Isaf).

There were 1,450 attacks in May, according to Isaf, surpassing the 1,400-mark for only the second time since January 2007. The last time monthly attacks topped 1,400 was last August.

More troops, more violence

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The spike has corresponded with the arrival of more US troops as part of Barack Obama's strategy on Afghanistan.

The US president is pouring thousands more troops into Afghanistan as part of a larger strategy to roll back Taliban influence and stabilise the war-torn nation.

Petraeus, who formerly commanded US troops in Iraq, is overseeing the military portion of the Obama strategy.

The US has already increased its military presence in Afghanistan to 56,000 troops, from about 32,000 in late 2008, and Petraeus said he expects to see a total 68,000 troops in the war zone in the second half of this year.

Analysts have warned that the Obama strategy could lead to sharply higher US casualty rates, potentially diminishing support for the war at home as members of congress head into mid-term elections in 2010.

Military officials blame the increase partly on a mild winter, which allowed Taliban and other fighters to cross freely into Afghanistan from havens in Pakistan.

But rising violence was also driven by heightened Nato and Afghan military operations, which increased by about one-third from January to May, Isaf said.

Civilian deaths

Meanwhile, the US military has launched an investigation into claims that civilians were killed in an air raid in Afghanistan's western Ghor province.

US troops are under pressure over civilians killed and wounded in its attacks [Reuters]
An Afghan official said 10 civilians, including five children, were killed in the attack in the Shahrak district on Tuesday, along with 12 Taliban fighters.

The US military said the attack was aimed at Mullah Mustafa, an alleged Taliban commander.

It initially claimed to have killed him, but said in a statement on Thursday that "credible reports surfaced that Mustafa survived the attack".

"A thorough review of intelligence and surveillance supports initial reports that all killed in the strikes were legitimate enemy targets," the statement said.

"In addition, we are working closely with Afghan partners to investigate unconfirmed reports of civilians among Mustafa's party."