American forces will expand deployment in the southern and eastern parts of Afghanistan to increase security so that reconstruction work can begin unmolested by Taliban and Qaeda attackers, the top military commander here said Sunday. The announcement by the commander, Lt. Gen. David Barno, amounted to an admission by the Americans that the 11,500 troops in Afghanistan have been unable to stop a constant stream of insurgent attacks that have undermined or slowed international aid efforts. The announcement also signaled a major shift in emphasis for the so-called provincial reconstruction teams run by the military, which have been helping mainly to provide emergency relief to Afghans and win the trust of the population. Now those teams will focus primarily on providing security in the southern and eastern areas of Afghanistan that have been most vulnerable to insurgent attacks this year. Full Story