The United States on Tuesday issued a veiled warning to Afghanistan's jilted defense minister not to undermine the country's fragile security.

The comments came a day after President Hamid Karzai (search) dropped Defense Minister Mohammed Fahim (search) from his election team, saying his running mate in Oct. 9 president elections would be a brother of late resistance hero Ahmad Shah Massood (search).

On Tuesday, U.S. Ambassador Zalmay Khalilzad (search) restated Washington's support for Karzai. But he acknowledged that his surprise decision to ditch Fahim had sown mistrust among leaders whose armies helped America rout the Taliban 2½ years ago.

Asked if the United States would intervene if the militias caused trouble, Khalilzad said the defense minister had a "direct responsibility" for preventing violence that has marred every change of Afghan president in living memory.

"We expect everyone to do everything they cannot to undermine stability here," Khalilzad said. "Political competition is good, it's fine. That's the way of the future in Afghanistan. But the use of force to resolve political disputes is a thing of the past."

The U.S. military leads about 20,000 troops in Afghanistan and has total control of its skies.

This year, American warplanes have circled over cities, including Herat and Chagcharan, in an attempt to calm battles between militias who Karzai says are now a bigger threat than the militants that the U.S. forces came to combat.

But the U.S. military has never taken on a powerful faction such as the Tajik forces controlled largely by Fahim, who marched victoriously into Kabul in late 2001 in defiance of U.S. orders and has resisted attempts to disarm them.

Fahim was conspicuously absent from a news conference Monday in which Karzai presented his team, including his candidate for first vice president, Ahmad Zia Massood.

Fahim has made no public comment.

Massood, currently ambassador to Russia, is a brother of Ahmad Shah Massood, who led the resistance to the Taliban regime until he was killed by Al Qaeda terrorists on Sept. 9, 2001. Karzai named Hazara leader Karim Khalili his choice for second vice president.

NATO troops have mounted extra patrols because of the rising political temperature, but both they and a U.S. military spokesman said there were no signs of unusual military activity.