Militants in Afghanistan have killed three American soldiers in a bomb and gun attack on their convoy.

A U.S. military statement said the convoy struck an improvised bomb before coming under small arms fire Thursday morning just north of Kabul.  An unidentified fourth coalition soldier was wounded in the attack.

The ambush was in Kapisa province, the site of Bagram, the main U.S. military base in Afghanistan.

U.S. President Barack Obama has ordered the deployment of 21,000 more troops and trainers to Afghanistan to fight the worsening Taliban insurgency and help with reconstruction.

Despite the arrival of more foreign troops, violence in the past year has increased to the highest levels since the U.S.-led invasion in 2001.

Some information for this report was provided by AP and Reuters.