KABUL,  (Reuters) – Seven U.S. troops were killed in  two separate roadside bomb attacks in southern Afghanistan yesterday, the NATO-led International Security Assistance Force  (ISAF) said.

Violence in Afghanistan is at its worst since the Taliban  were overthrown by U.S.-backed forces in late 2001, with  soaring casualty rates among foreign and Afghan troops as well  as among civilians who are caught up in the conflict.

Foreign military casualties have reached record levels this  year, with more than 470 troops killed so far compared with 521  for all of 2009, according to website www.iCasualties.org and  figures compiled by Reuters.

Yesterday, ISAF said five of its troops were killed by a  roadside bomb in one incident in southern Afghanistan, while  two others died in a separate bomb attack, also in south.

An ISAF spokesman said all seven were Americans.

Earlier, a Reuters witness and residents reported seeing a  U.S. military armoured vehicle ablaze after it was hit by a  roadside bomb in Kandahar city in the south.

It was not immediately clear whether the Kandahar incident  was one of the bombings referred to by ISAF.

The latest troop casualties bring to 14 the number killed  in the past three days, with ISAF reporting seven other troops  killed on Saturday and Sunday.

About 2,050 foreign troops have been killed since the war  began, more than 60 percent of them Americans. Of those, at  least 259 have died in the past three months.