(Rewrites throughout)

By Kristin Roberts

WASHINGTON, Nov 8 (Reuters) - Pentagon officials say Pakistan's nuclear arsenal is secure in military hands, but some U.S. lawmakers and experts warn that nuclear material and designs could leak out if political instability persists.

Pakistan's president, Gen. Pervez Musharraf, has concentrated control over the entire nuclear program.

But a decline in his support within the military amid the current political crisis raises a risk that control over the weapons could weaken. That could open the door to theft or sale of weapons material to extremist groups, some experts say.

Some weapons experts and U.S. officials still suspect Pakistan's military of at least knowing about the smuggling activities of Pakistan's A.Q. Khan network that sold weapons technology to Iran, North Korea and Libya.

"This is a country that's leaked nuclear weapons designs, centrifuges," said David Albright, president of the Institute for Science and International Security. "Two of its scientists talked to (Osama) bin Laden about how to make nuclear weapons in 2001."

"It's a system that's leaked very dangerous information," he said. "You have to worry about the integrity of the system in a period of growing instability."

A senior U.S. general this week said the Pentagon was worried about the security of Pakistan nuclear weapons after Musharraf declared a state of emergency on Saturday, prompting protests and arrests.

But on Thursday, other U.S. defense officials backpedaled, saying the weapons are under the military's full control. They said the Pakistani military was a responsible steward of the arsenal and would stay out of the current political conflict.

"From the Army's standpoint, from the military's standpoint, the confidence remains high," one U.S. military officer said.

"There has been no break in that control," the officer said about the Pakistani military's control over the weapons.

U.S. lawmakers, however, expressed more uncertainty about the security of the weapons.

Rep. Ellen Tauscher, a California Democrat on a congressional committee that oversees the U.S. military, said the United States lacked full knowledge of Pakistan's weapons.

"We need a lot more visibility on what's going on in Pakistan. Who does have that football? Who is next in line?," she said about Pakistan's nuclear weapons.

"I've learned that we don't have as strong a handle on it," said Tauscher, who has access to some U.S. intelligence as a member of the House of Representatives Armed Services Committee.

Asked if she knew specific details of Pakistan's weapons, including their location and the chain of command over those weapons, she said, "I don't know, and I've asked the question a couple of different ways."

Sen. Joseph Biden of Delaware, a Democratic presidential hopeful and chairman of the Senate Foreign Relations Committee, said Pakistan was at real risk of becoming a failed state.

"I think it's very real," he said of that threat.

Still, global financial markets have largely failed to react to the political unrest in Pakistan this week. Market sources say they remain more concerned about uncertainty elsewhere, particularly in Iran, but note that could change quickly if the crisis deepens. (Editing by Mohammad Zargham)

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