A U.S. air attack killed six rebels in southern Afghanistan (search) in an exchange of fire after militants shot a rocket at an American helicopter, the U.S. military said Tuesday.

Militants fired a rocket and small arms at a Black Hawk helicopter on Monday in Zabul province, the military said. No American troops were injured and there was no damage to the helicopter before another chopper — an Apache gunship — "engaged the enemy, killing six insurgents," the statement said, without elaborating.

Also Monday, two U.S. soldiers on a routing security patrol were wounded when a homemade bomb destroyed their Humvee in Shinkay, another district of Zabul close to the Pakistani border, the military said. It said the two wounded soldiers would be flown to Germany for treatment but did not identify them or describe their condition.

Two American soldiers were killed Monday in a battle with insurgents in neighboring Paktika province (search), part of tract of south and eastern Afghanistan where militants have led a stubborn insurgency since the Taliban (search) were ousted in 2001.

The attacks were the latest evidence militants are stepping up violence ahead of Afghanistan's landmark elections Oct. 9.

The blast that wounded the two soldiers in Shinkay was caused by a freshly laid mine, Zabul Gov. Khial Mohammed said, blaming rebels from the former ruling Taliban for the attack. He said Afghan troops were scouring the area, but had found no suspects.

The military also said three rockets were fired at a U.S. base in Paktika, but landed 50-100 yards away, injuring no one. A roadside bomb attack on a convoy in the eastern city of Jalalabad also caused no injuries, it said. It was not immediately clear when those two incidents took place.

President Hamid Karzai (search) and one of his vice presidents have both escaped attacks in the past week, part of a wave of violence that officials predict will surge ahead of the country's first-ever direct presidential election.