Three US soldiers were killed in attacks in southern Afghanistan, the military said Thursday, with security forces on alert for Taliban attacks during voting in presidential and provincial elections.

The new deaths, all on Wednesday, added to a mounting death toll for international troops in Afghanistan that has reached 50 so far this month, according to a toll maintained by the independent icasualties.org group.

Two were killed in a small arms attack on a patrol in the south, the International Security Assistance Force said in a statement. The third was killed in a separate hostile attack, it said.

The US military confirmed all were US nationals. It did not say in which provinces the troops lost their lives.

There are about 62,000 US forces in Afghanistan, making up the bulk of an international deployment from 42 countries.

The international troops were on alert for Thursday''s election but were operating behind the scenes, leaving Afghan forces to man the polling stations.