U.S. missile strikes in Pakistan's tribal areas in recent months have killed three of the top 20 extremist leaders there, causing a blow to insurgents threatening nuclear-armed Pakistan's very existence, a top U.S. general said Wednesday.

Gen. David H. Petraeus, the new chief of U.S. Central Command, said controversial airstrikes launched into Pakistan's unruly tribal areas in the last three months were a topic of conversation with every Pakistani leader he met this week. Pakistani leaders have criticized the missile strikes as a violation of their sovereignty.

"Certainly there does have to be a better explanation of the blows that have been struck in recent weeks and months," Petraeus told The Associated Press in an interview. "It is hugely important that three of 20 extremist leaders have been killed in recent months."

Petraeus did not identify the extremist leaders he said died in the U.S. strikes.

There have been more than 17 reported airstrikes since August in Pakistan's tribal areas, a region awash in Taliban and Al Qaeda militants who launch attacks in both Afghanistan and Pakistan.

The Pakistani government has complained bitterly about the cross-border missile strikes, and on Tuesday Prime Minister Yousuf Raza Gilani warned the next U.S. president must halt the attacks or risk losing the war against Al Qaeda and the Taliban.

The prime minister said the U.S. should share intelligence with his country's military to allow Pakistan to go after militants themselves.

Washington says Pakistan's tribal areas are believed to be home to many of Al Qaeda's top leaders, including Usama bin Laden, and they may be planning another big attack on the West.

Speaking at the sprawling Bagram military base north of Kabul, Petraeus described the insurgents on both sides as a "mutual enemy," who in the case of Pakistan represent "an existential threat, and they recognize it as such."

"When I was in Islamabad, in Peshawar, I was very impressed by the determination of Pakistani leaders indeed to take steps to deal with what they see as a threat to their very existence posed by the extremists in the Federally Administered Tribal Areas and in some other areas of their country," Petreaus said.

Petraeus, who became U.S. Central Command chief on Oct. 31, has been credited for turning the tide of violence in Iraq, and many expect Afghanistan will see some of the same tactics, such as co-opting local tribal leaders to resist the Taliban.

While acknowledging the security situation in Afghanistan has deteriorated in the last year, Petraeus said Afghanistan's government is looking at new initiatives to engage Afghan tribes in the fight against insurgents, a similar tactic to the one that helped bring down the levels of violence in Sunni areas in Iraq.

"That discussion is bubbling up, if you will, and I think that there are some very thoughtful approaches that are being looked at as options," Petraeus said without disclosing any details of the initiatives under discussion.

"This is a country in which support of the tribes, of the local communities, for the overall effort is essential. It is a country that has not had a tradition of central government extending into the far reaches of its provinces and its districts," he said.

"So it is essential that, again, the various tribes, various communities indeed oppose the extremism, oppose the insurgents and enable the effort of the Afghan government and the coalition," Petraeus said.