MUZAFFARABAD, Pakistan (CNN) -- Pakistan took steps to protect the capital of its portion of Kashmir from possible Indian airstrikes and moved more troops to its Indian frontier Friday amid growing fears of war between South Asia's nuclear rivals.

Both India and Pakistan stepped up their preparations for a possible conflict over Kashmir, the Himalayan territory that each has claimed since independence from Britain in 1947. (History)

Local authorities said Friday the city of Muzaffarabad, the capital of Pakistani-administered Kashmir, will be under a nighttime blackout from 11 p.m. to 5 a.m. India says the city is home to camps that train Kashmiri separatist fighters battling Indian rule over its portion of Kashmir, and authorities in Muzaffarabad said an Indian air raid on the city is possible.

Hundreds of thousands of Indian and Pakistani troops are amassed along the border between the two countries and along the Line of Control, the U.N.-drawn border dividing Kashmir between India and Pakistan.

India and Pakistan have fought three wars since independence, two of them over Kashmir. Both countries tested nuclear weapons in 1998 and have developed medium-range missiles capable of striking deep into each other's territory. U.S. officials say a nuclear war in south Asia could leave up to 17 million dead and wounded in the first weeks of an all-out exchange. (Full story)

Pakistani President Pervez Musharraf in January banned two groups India blames for the violence, but U.S. and British officials have urged Musharraf to do more to defuse the crisis. Musharraf said Friday that Pakistan will not initiate a conflict, but "I think you need to get assurances from the Indian side."

"All I can do is to give my own assurance that we will try to avoid conflict. It will be my utmost endeavor to avoid conflict," he said.

Despite the continuing buildup, India's defense minister described the situation as "stable" Friday. (Full story)

India has threatened military action to stop Kashmiri fighters it says are armed and trained by Pakistan. Pakistan denies providing more than moral and diplomatic support for the militants.

The two countries have amassed about 1 million troops on their border and along the Line of Control dividing Kashmir since a December attack on India's Parliament that New Delhi blames on Kashmiri militants. The crisis intensified with an attack on an Indian army base in Kashmir in early May that killed 34 people, mostly women and children. (Timeline)

But U.S. officials urged Americans to leave India on Friday, and the British government said its citizens should consider leaving as well. Both governments authorized the departure of non-essential embassy staff and dependents: Similar steps already had been announced for Pakistan. (Full story)

U.S. officials have indications that Pakistani authorities had ordered a halt to incursions into the Indian-ruled portion of Kashmir, State Department spokesman Richard Boucher told reporters Friday. But Pakistan disputed Boucher's comments, saying there had been no incursions and no order to stop them.

"Pakistan has categorically stated it will not allow its territory to be used for the export of terrorism," said Maj. Gen. Rashid Qureshi, a spokesman for Musharraf. "This statement that Pakistan has passed instructions to these people to stop incursions is strange."

The India-Pakistan crisis could deal a serious blow to the U.S.-led antiterrorist campaign in Afghanistan, where American and British troops have been working with Pakistani forces to search for al Qaeda and Taliban leaders along the rugged Afghan-Pakistani border. The Bush administration has become deeply involved in efforts to resolve the standoff, with Defense Secretary Donald Rumsfeld and Deputy Secretary of State Richard Armitage headed for the region next week.

U.S. officials believe al Qaeda members may be trying to foment conflict between India and Pakistan to distract the Pakistani government from pursuing the terrorist network.

While they say they have no evidence connecting al Qaeda to attacks on Indian targets, officials say evidence of ties to militants suspected of involvement is considerable. Pakistani officials say -- and U.S. officials confirm -- that some al Qaeda members have moved into major cities throughout Pakistan.

Brahma Chellaney, a professor of strategic studies at the Center for Policy Research in New Delhi and an informal adviser to the Indian government, told CNN that "war clouds will begin to disperse" once Musharraf cracks down on suspected terrorists within Pakistan.

"If that does not happen, war is very likely," he said. "Unless Pakistan cracks down on terror groups, there can be no progress in the war against terror. You can wipe out al Qaeda in Afghanistan and finish the Taliban, but as long as the terror infrastructure of Pakistan remains, you'll have jihadis -- Islamic holy warriors -- produced from the religious schools and terror training camps of the country."