A bomb hit a U.S. military vehicle Monday in central Afghanistan, killing four American troops, the military said.

The attack occurred in Dihrawud district, Uruzgan province, as the service members were patrolling in an armored Humvee, the military said in a statement.

Shortly after the blast, militants fired at the troops with guns and rocket-propelled grenades. The military responded, calling in attack helicopters and fighter planes, it said.

An assessment of militant casualties is ongoing, the statement said.

"This is a sad and tragic day for us all," said Brig. Gen. John Sterling, a U.S. commander.

The names of the victims was withheld pending notification of next of kin.

The bombing brings the death toll of U.S. personnel to 214 in and around Afghanistan since the U.S. invaded the country in late 2001.