The top US commander in Afghanistan said he wants to extend the combat tours of 1,200 soldiers amid rising violence here, and US Defense Secretary Robert Gates said Pakistan must act to stem an increasing flow of Taliban fighters across the border. The prospect of a troop increase in Afghanistan, as US President George W. Bush orders 21,500 more troops into Iraq, raises questions about the military's ability to sustain its war-fighting on two major fronts. About 24,000 US troops are deployed in Afghanistan, the most since the war began in October 2001, according to Lt. Gen. Karl Eikenberry, the senior American commander in the country. It also raises questions about the future course of the war in Afghanistan, which the United States is increasingly handing off to NATO forces. Of the 31,000 troops here under NATO command, about 11,000 are American. The remainder are hunting down al-Qaida terrorists and training the Afghan army.