KABUL, Afghanistan — U.S. troops and helicopter gunships killed 22 militants, including three Arab fighters in southern Afghanistan, the military said Monday, the latest bloodshed ahead of historic Afghan elections.

Meanwhile, the United Nations withdrew dozens of staff from the western city of Herat a day after mobs ransacked its offices. The mob violence came after President Hamid Karzai fired the city’s warlord governor. His replacement later ordered a 9 p.m. curfew.

The 12-hour battle in the southern province of Zabul, a hotbed of resistance to Karzai’s U.S.-backed government, began late Sunday, the military said.

Spokesman Maj. Scott Nelson said some 40 militants attacked coalition soldiers on a search operation. The troops called in two Apache helicopters, which opened fire on the fighters.

In another incident Sunday, Nelson said Taliban gunmen ambushed a coalition patrol near the southern city of Kandahar.