LONDON – An audacious Taliban attack on Pakistan's army headquarters shows there is a growing terrorist threat to the nuclear-armed U.S. ally, Secretary of State Hillary Rodham Clinton said Sunday.

But Clinton and her British counterpart said there was no risk of Pakistan's nuclear arsenal falling into terrorist hands.

Clinton said extremists were "increasingly threatening the authority of the state, but we see no evidence that they are going to take over the state."

"We have confidence in the Pakistani government and military's control over nuclear weapons," she said.

At a joint news conference, British Foreign Secretary David Miliband said Pakistan faced a "mortal threat," but there was no danger of Pakistan's nuclear weapons being seized by terrorists.

"I think it's very important that alarmist talk is not allowed to gather pace," he said.

Taliban militants have launched a series of increasingly bold attacks on military and political targets in Pakistan. On Saturday, militants dressed in military fatigues attacked the army headquarters in Rawalpindi, taking dozens of hostages.

The 22-hour siege ended Sunday when commandos stormed the building. At least 19 people died in the standoff, including three captives and eight of the militants.

Clinton is on a five-day tour of Europe in which the conflict in Afghanistan and neighboring Pakistan and the crisis over Iran's nuclear program have been major topics.

Miliband said his talks with Clinton in London focused on Afghanistan. Britain has 9,000 troops in the country, but support for the mission has waned in recent months amid rising casualties and allegations of major fraud in August's Afghan national elections.

Miliband said the U.S. and Britain had a "shared strategy" that involved building Afghanistan's civilian and political institutions as well as defeating the Taliban.

Clinton also warned Iran that the world "will not wait indefinitely" for it to live up to international obligations regarding its nuclear program.

The U.S. and its allies fear Iran wants to build an atomic bomb and is using what it calls a civilian nuclear energy program as cover. Iran is under U.N. penalties for refusing to stop enriching uranium, an important first step toward building a bomb.

Clinton later held talks with British Prime Minister Gordon Brown at Chequers, the prime minister's country retreat outside London. She said the trans-Atlantic "special relationship" remained strong.

"I have a special personal relationship with the prime minister and of course I think it can't be said often enough, we have a special relationship between our countries," she said.