Four U.S. troops killed in southern Afghanistan

U.S. Army soldiers from Gator Company, 2-12 Infantry, 4th Brigade run for cover from small arms fire following an attack on a transport convoy in the Pesh Valley in Afghanistan's Kunar Province August 4, 2009.

KABUL (Reuters) - A roadside bomb killed four U.S. troops in Afghanistan's southern Helmand province, where thousands of Marines are conducting the biggest operation of the war, the military said on Friday.

The death toll is on the rise among Western troops in Afghanistan. Last month, 76 foreign troops died in Afghanistan, making it by far the deadliest month of the war. This month 15 have already died, including 11 Americans.

A U.S. military spokesman in Kabul said the four were killed when their vehicle struck the homemade bomb on Thursday. He gave no details of the location of the incident or the unit to which the troops belonged.

There are now more than 100,000 Western troops in Afghanistan, including about 62,000 Americans -- nearly double the U.S. strength at the start of the year as President Barack Obama has sought to turn the tide in the eight-year-old conflict.

Insurgents have stepped up attacks ahead of a presidential election on August 20, which the Taliban have vowed to disrupt.

Homemade bombs are by far the insurgents' most lethal weapons, scattered throughout the province where 4,000 Marines surged forward into the lower Helmand River valley last month, a Taliban stronghold of opium poppy fields.

Violence across Afghanistan has hit its worst level since the Taliban were ousted in 2001 and has intensified since the Marines launched their Operation Strike of the Sword offensive to seize Taliban-held areas before the vote.

British troops in Helmand have also launched a huge, simultaneous offensive in another part of Helmand, Operation Panther's Claw, facing heavy losses as they seize territory north of the provincial capital Lashkar Gah.

Military commanders had warned of heavy casualties ahead of the Helmand offensives, part of Obama's new regional strategy to defeat the Taliban and its allies and stabilize the country.

(Reporting by Hamid Shalizi; Editing by Nick Macfie)