Three U.S. bombs went astray in Afghanistan over the weekend, landing in a civilian neighborhood near Kabul and near a senior citizens' center in Herat, Pentagon spokeswoman Victoria Clarke said Tuesday.

A helicopter trying to recover the remains of another U.S. helicopter that crashed Friday night was shot at while refueling at an airfield in Pakistan Saturday, Clarke said. No U.S. forces were hurt in that incident, Clarke said.

An F-14 attack plane dropped two 500-pound bombs on a residential area northwest of Kabul on Saturday while aiming at military vehicles a half-mile away, Clarke said. She said the military did not have any information on casualties caused by those bombs.

On Sunday, an F/A-18 dropped a 1,000-pound bomb near a senior citizens' home in the northwestern Afghan city of Herat, Clarke said. The bomb landed in a field between the home and a military vehicle storage facility, she said. The two buildings are 300 feet apart.

In Pakistan, United Nations spokeswoman Stephanie Bunker, citing independent U.N. sources in Afghanistan, said a bomb hit a military hospital within a military compound on Herat's eastern edge. U.N. officials did not know whether the hospital was being used at the time, or whether any civilians or military personnel were hurt, Bunker said.

The ruling Taliban government has claimed that the United States bombed a hospital in Herat, killing 100 people.

Clarke said she did not know if the building she called a senior citizens' center was the same as the hospital involved in the reports from the United Nations and from the Taliban.

Defense Secretary Donald H. Rumsfeld said Monday there was no evidence that a hospital had been hit.

Meanwhile, warplanes that started attacking Taliban front-line troops north of Kabul over the weekend may have to keep up the pounding for weeks to dislodge the dug-in embattlements, military analysts said.

American warplanes have increasingly focused on Taliban troops and equipment on the front lines of the civil war between the Taliban militia and a loose network of opposition forces.

Despite the airstrikes, however, the opposition northern alliance needs more military help before moving on the Afghan capital, the alliance's Washington representative, Haron Amin, said Monday.

"It is better than other days, but a lot more of it is needed for us to make ground moves," Amin said.

Military analysts agreed that airstrikes against Taliban forces could continue for weeks.

"I don't think it's a symbolic action. I think it's a real military requirement" to destroy the Taliban's forces, said Michael O'Hanlon, a Brookings Institution expert on defense.

Pentagon officials have said up to 15,000 Taliban troops appear to be entrenched in a labyrinth-like complex of caves, trenches and bunkers north of the Afghan capital. Those forces have impeded advances over the years by the opposition northern alliance.

Sunday, at the beginning of the third week of U.S.-led airstrikes, American aircraft shifted their focus from hitting Al Qaeda terrorist camps and Taliban centers elsewhere to the ruling militia's troop concentrations north of Kabul.

"The reason for the air attacks on Taliban and Al Qaeda forces is to destroy Taliban and Al Qaeda forces," Defense Secretary Donald H. Rumsfeld said Monday. He denied suggestions that the Bush administration had held back bombing the front-line Taliban positions out of fear that the northern alliance would capture Kabul before a replacement government acceptable to Afghanistan's many competing factions could be established.

The core of the Taliban's support comes from Pashtun tribesmen, a major ethnic group in neighboring Pakistan as well. Pakistan, a key U.S. ally in its global war on terror, opposes an alliance takeover in Afghanistan, because its main support comes from other Afghan ethnic groups.

Air Force Gen. Richard Myers, chairman of the Joint Chiefs of Staff, told Monday's Pentagon news conference that earlier attacks were designed to undercut the Taliban's ability to replace troops it loses on the front lines. He said the attacks destroyed many Taliban transport aircraft used either to fly reinforcements to the front lines or to extract wounded troops.

In explaining the timing of the move against Taliban front-line fighters, Myers emphasized the payoff for the northern alliance.

"We're starting to work on some Taliban targets that are arrayed out in the field against folks that we would like to help, and that's what we're about," Myers said.

The front-line attacks are crucial to aiding the northern alliance because it is not strong enough to defeat the Taliban on its own, argued Brookings' O'Hanlon.