Chief of U.S. Central Command General David H. Petraeus has said 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. In an interview to a foreign news agency, he 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 said. "It is hugely important that three of 20 extremist leaders have been killed in recent months".