ISLAMABAD, Pakistan - An unusually large barrage of missiles fired by remotely piloted U.S. drones killed 16 people in the tribal area of North Waziristan yesterday, a possible indication that the U.S. plans to escalate such attacks after Pakistan declined to step up its operations there.

The attacks came in a week when top U.S. military officials visited Islamabad and asked that Pakistani authorities do more to go after insurgent groups that are based in North Waziristan but are focused on killing U.S. troops across the border in Afghanistan.

Pakistani officials say that their military is stretched thin by an operation in South Waziristan and that now is not the time to expand the campaign into the adjacent territory to the north. U.S. authorities have countered that if Pakistan does not go after the groups, the United States will.

News of the attacks broke as Pakistani President Asif Ali Zardari, a U.S. ally, faced new challenges to his authority. Pakistan's Supreme Court ruled on Wednesday that a broad amnesty agreement that had protected Zardari and thousands of other government officials was unconstitutional. Yesterday, the president was met with growing calls to resign from opposition politicians, and analysts said his position is increasingly precarious.

Top deputies in his government are also under threat: The national anticorruption agency said it had placed more than 200 officials on a list that bans them from leaving the country because of alleged crimes that had been wiped clean by the amnesty but are now subject to prosecution.

Interior Minister Rehman Malik was on the list, as was Defense Minister Ahmed Mukhtar, who called local television stations late last night to say he had been barred from boarding a flight to China.

Zardari, who has denied any wrongdoing, was reportedly not on the list, but the travel ban for top officials will make it difficult for his government to function.

His troubles complicate U.S. efforts to enlist greater Pakistani participation in a plan to squeeze insurgent groups that operate in the remote Afghan-Pakistan border region.

The U.S. government does not officially acknowledge its program of drone attacks, which is its primary tool for going after militants who are based on the Pakistani side of the border. The government here officially condemns the attacks, but cooperates with the CIA in gathering intelligence needed to carry out the strikes.

Yesterday's attacks included one set of strikes that officials said involved 10 missiles fired from five drones - an unusually heavy concentration of firepower on a single target.

Officials said 15 people died in those strikes, including a well-known al-Qaeda commander named Zuhaib al-Zahibi, while one person was killed in an earlier strike.

North Waziristan is the suspected home of the top al-Qaeda leadership, as well as the network of Afghan insurgent leader Siraj Haqqani, who is believed to be behind some of the most potent attacks against U.S. troops in Afghanistan.