Four soldiers with U.S.-led coalition forces and an Afghan civilian working for them were killed in southern Afghanistan on Tuesday when their vehicle struck a roadside bomb, the U.S. military said.

It said the coalition troops were on a patrol with Afghan security forces, but did not give any further details.

Some 38,000 U.S. troops are currently serving in Afghanistan alongside another 30,000 from 40 other mostly NATO nations.

Around half the U.S. troops serve under a NATO-led force, while the rest operate under a U.S.-led coalition. Almost all troops in the coalition are American.

Many coalition troops are involved in training and mentoring Afghan security forces, especially in the south and east of the country where the Taliban insurgency is strongest.

President Barack Obama last week ordered 17,000 more U.S. troops deployed in Afghanistan to reinforce mainly British, Canadian and Dutch forces in south of the country who are locked in a stalemate with the Taliban insurgents there.

Commanders predict violence will rise in Afghanistan this year as the new troops venture into new areas of the south and try to enforce security ahead of presidential polls on Aug. 20.