A homicide bomber struck U.S. troops patrolling on foot in southern Afghanistan on Thursday, killing three civilians and wounding at least nine others. The Americans suffered casualties but it was not immediately clear how many.

The blast followed an allegation from President Hamid Karzai that 17 civilians were killed during clashes between U.S.-led troops and insurgents earlier in the week. The U.S. military insists all 32 people killed in the fighting were militants.

Thursday's strike against the U.S. patrol occurred on a busy street lined with shops in Kandahar province's Maywand district, said district chief Naimatullah Khan. Helicopters took away American casualties, said Khan, but he could not provide any numbers or say whether they were killed or injured.

Col. Jerry O'Hara, a U.S. military spokesman, confirmed U.S. casualties as a result of the bombing, but he too could not give further details.

A Taliban spokesman Qari Yousuf Ahmadi claimed responsibility for the blast in a phone call to an AP reporter in southern Afghanistan.

The Taliban regularly use homicide attacks against Afghan and foreign troops, but the majority of victims are civilians. There were 1,160 civilians killed in insurgency-related incidents in 2008, according to an AP casualty count — 368 by foreign and Afghan troops and 768 by the Taliban. Another 26 were caught in crossfire.

Civilian deaths at the hands of coalition troops are a major source of friction between Karzai and foreign troops here, and the latest claims threatened to worsen the tensions.

In a statement, Karzai blamed the "terrorists" for using civilians as human shields in their battles with foreign troops, but he also criticized international forces for engaging insurgents in Afghan villages.

Meanwhile, Australia's Defense Force said Thursday it too was investigating allegations that its troops killed or wounded civilians in central Uruzgan province during a series of battles between Taliban insurgents and NATO forces. Australia has about 1,000 troops in Afghanistan.

Afghanistan's Interior Ministry said 11 civilians were killed in the fighting Sunday and Monday. NATO has said it was only able to confirm nine injured civilians.

Separately, NATO said Thursday a roadside bomb killed one of its soldiers in Kandahar province the day previous. It did not identify the soldier's nationality.

Violence in Afghanistan has skyrocketed in the last two years, and the U.S. has said it will deploy an extra 20,000 troops to combat the insurgents.

More U.S. troops, 151, died in Afghanistan in 2008 than any other year since the 2001 invasion to oust the Taliban for harboring Osama bin Laden after the Sept. 11 attacks. U.S. officials have warned the violence will likely intensify in the coming year.