Secretary of State Hillary Rodham Clinton warned in unusually bleak terms Wednesday that Pakistan's fragile government is facing an "existential threat" from Islamic militants who are now operating within a few hours of the capital.

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Clinton spoke as militants expanded into new territory adjacent to the Swat Valley and 60 miles from Islamabad, and top U.S. officials continue to make regular visits to Pakistan. Adm. Michael G. Mullen, chairman of the Joint Chiefs of Staff, is visiting now. A special U.S. envoy, Richard C. Holbrooke, was there this month.

Clinton's comments to the House Foreign Affairs Committee underscored increasing U.S. alarm at how the militants' strength has grown even as the Obama administration has begun trying to implement a new strategy for stabilizing the country. U.S. officials are worried not only about the stability of Pakistan, but also about neighboring Afghanistan, where they are committing an extra 21,000 troops this year to try to stanch the advances of the Taliban and allied insurgents.