KABUL, August 25, 2009 (AFP) - A roadside bomb blast killed four US soldiers in Afghanistan on Tuesday, making 2009 the deadliest year recorded for foreign troops in the country since the US-led invasion ousted the Taliban in 2001.

The grim milestone was likely to further fuel political debate in Washington, London and the capitals of other NATO troop contributing countries where the escalating war is increasingly being questioned.

"Four International Security Assistance Force service members were killed today as a result of an improvised explosive device (IED) detonation that occurred in southern Afghanistan," NATO announced in a statement.

All four soldiers were American, the military announced.

IEDs are the weapon of choice for Taliban insurgents fighting the Western-backed Afghan government and foreign troops, and are particularly strong in the south.

The latest deaths came as incumbent Hamid Karzai was narrowly leading a neck-and-neck race for the Afghan presidency with just two percent more votes than his closest rival, former foreign minister Abdullah Abdullah.

The partial results amount to about 10 percent of the total ballots cast in only Afghanistan's second direct presidential election, which was held last Thursday and has been overshadowed by claims of massive fraud.

The independent icasualties.org website said 63 foreign soldiers died in Afghanistan so far this month and 295 so far this year, making 2009 the worst year on record after less than nine months.

This year, the same website said, 172 US troops have died so far, up from 155 US troops in 2008 out of total foreign troop deaths of 294.

US President Barack Obama has made Afghanistan the cornerstone of his foreign policy and ordered an extra 21,000 troops to the country in a bid to turn around the war against the Taliban, whose insurgency is at record levels.

Foreign troop casualties have risen steadily each year and the Taliban-led insurgency is now claiming record deaths.

Obama's hand-picked commander of US forces in Afghanistan, General Stanley McChrystal, is expected to call for even more troops in a formal review of the war-fighting strategy anticipated within the next two weeks.

The decision will likely be wrenching for Obama particularly as a large proportion of the US public has turned against the war and his popularity has waned.

The president, who accused predecessor George W. Bush during the 2008 campaign of favouring a "war of choice" in Iraq over a "war of necessity" in Afghanistan, has already ordered 21,000 more soldiers to the fight.

That could prove to be a major political headache for an administration locked in pitched battles for congressional approval of its sweeping domestic agenda, including a health care overhaul and measures to fight climate change.

Britain, Canada and the Netherlands are the biggest troop contributors to the NATO-led effort in Afghanistan, after the United States.

British casualties have already shot beyond the 200 milestone to 206, renewing debate about the country's role in the conflict, the equipment available to protect its troops and whether any progress is being achieved.

The Afghan government said Tuesday that four civilians were killed and five others were wounded in a roadside bomb, in the east near the Pakistan border.

The bomb that killed the Afghan civilians was similar to that which killed the US troops. The government blamed the attack in Paktia province on Taliban-linked rebels.