U.S. warplanes bombarded Taliban positions just behind the front line north of Kabul on Sunday in what could be the start of a more aggressive campaign to help the northern-based alliance fighting the ruling Taliban militia. 

There had been limited U.S. bombing of areas behind Taliban front lines near Kabul in recent days, but Sunday's bombardment appeared to be the heaviest and closest to the front line. 

The U.S. bombs hit about a mile behind the front line in the district of Kohesafie, 25 miles north of Kabul, Abdul Ghafur, an opposition fighter who was in radio contact with front line commanders, told The Associated Press. 

Several eyewitnesses, including journalists and residents, reported Taliban front line positions bombed in the area. 

Several jets roared high in the skies above the opposition-held Panjshir Valley north of Kabul, the Afghan capital. Residents stopped their daily activities and stared at the screaming planes overhead. 

Northern alliance officials had been asking the United States to bomb the front line north of Kabul so that they could advance on the capital. But until now U.S. bombing of front line positions has mostly taken place around the strategic northern city of Mazar-e-Sharif, not Kabul, apparently because of reservations about the northern alliance taking national power. 

At this position near the front line north of Kabul, opposition fighters said they were hoping the United States would soon bombard Taliban forward positions. 

The apparent U.S. reluctance to move more aggressively against front line targets near Kabul may reflect American concern about a possible rapid advance by opposition forces on the capital before a political agreement is reached on the makeup of a post-Taliban government. 

A national government dominated by the northern alliance could be problematic because the alliance represents mostly minority interests in the north and because its leaders ran a chaotic and violent administration when they were in power five years ago. 

The Taliban are the target of U.S. commando and air attacks because they harbor Usama bin Laden and the Al Qaeda terrorist network, accused of masterminding the Sept. 11 terror attacks on the World Trade Center and the Pentagon.