INSIDE the low, white building, at the end of a broad avenue in the heart of the capital, the president and his advisors agonise over decisions which could lead to a new world order.

But this is not George W Bush in the White House; instead it is thousands of miles away, in the presidential palace at the end of Constitution Avenue in Islamabad, Pakistan.

Pakistan's leader, General Pervez Musharraf, has become a key figure in US plans for retaliation against Osama bin Laden, the man suspected of masterminding the terrorist attacks on the World Trade Centre and the Pentagon last week. His regime's close links with the Taliban in neighbouring Afghanistan, who provide shelter for bin Laden, and deteriorating relations with the West, have made Pakistan's support for any international action, or at least its acquiescence, a crucial factor.

And, added to this, is the complication that, since carrying out tests in 1998, Pakistan has become the world's seventh declared nuclear power. Any action which causes further instability in the region could have far-reaching consequences.

In the immediate aftermath of last week's atrocity, General Musharraf offered "unstinting co-operation" to US efforts to combat terrorism. But this could put the Pakistani government at risk of fuelling Islamic militants inside its own borders.

Relations between Pakistan and the West had been close, helped by its hostility to India, seen as aligned with the Soviet Union during the Cold War, over the disputed territory of Kashmir. But the coup which brought General Musharraf to power, ousting an elected government, and the nuclear tests, have combined to alter the complex balance of alliances in the Middle East.

'The nuclear tests have been a major cause for concern for the West, and that has meant relations have been strained," says David Campbell, professor of international relations at Newcastle University. "Prior to that, relations were extremely close, but the tests went against the West's desire to prevent nuclear proliferation, and made things very difficult.

"Pakistan has also had a close relationship with the Taliban over a long period of time. It started out as an anti-Soviet position, which was consistent with their relations with India and the occupation of Afghanistan.

"A lot of the western aid and support for the mujahadeen to fight the Soviet occupation came through Pakistan, and a lot of the Taliban fighters trained in Pakistan. Pakistan's security and intelligence services have the closest relationship with the Taliban, which means the United States is interested in keeping Pakistan on-side," Prof Campbell says.

Despite the links - Pakistan is one of only three countries to recognise the Taliban regime - General Musharraf joined in international condemnation of the attacks on New York and Washington. But, in offering co-operation, there is the question of how far to go.

There are reports that US forces could operate from within Pakistan, but this risks inciting hostility to General Musharraf's government. More likely is that US jets will be given permission to fly over Pakistani airspace on the way to bombing missions in Afghanistan.

"The Americans will want to persuade Pakistan into providing as much support as they can, but I think they will probably be cautious about pushing them too far, over insisting on landing ground troops in Pakistan," says Prof Campbell. "Not only will that have a potentially unsettling effect on Pakistan itself, but they also want Pakistani intelligence to help them.

"It depends on what the Americans come up with. There is no doubt that there will be a major military action, that is a political necessity, although it is unlikely to be effective and runs the risk of being counter-productive.

"But I suspect it will be substantially the use of cruise missiles and other air power, followed most likely by the landing of special forces. I would be surprised if it involved large-scale infantry and ground troops." Allowing US planes to overfly Pakistani territory would still be a tough decision, although General Musharraf has indicated that a UN Security Council resolution authorising specific action would make that choice easier to justify.

And, in return for his co-operation, General Musharraf can expect something in return.

'There will be a lot of deals and a lot of trade-offs. You do one thing, you pocket a favour," adds Prof Campbell. "A few months down the road, and maybe Kashmir turns nasty, the Americans are not in a position to tell Pakistan what to do. I would not be surprised if some sanctions weren't lifted - international politics is full of this kind of horse-trading. They don't have to get anything back now, but they could expect something in the future."

But, as well as the possibility of US support in the future, General Musharraf's initial response was also a reaction to the terrorist outrage itself, according to Anoush Ehteshami, professor of international relations at Durham University.

"They are supportive because of the sheer scale of this tragedy, and, in the face of what the world saw, it is very difficult for the Pakistan government to do anything else," he says. "But they face a lot of pressure from Islamic organisations, and, since the end of the Afghan war, Pakistan has been the main place for access to the Taliban and a lot of militants are based there.

"Being a military government, they have the assets of the military at its disposal, but that also could become problematic. The government is not unpopular - it put an end to a very corrupt government - but if the Taliban put pressure on, let off a few bombs and assassinate a few people, that could change. If the government in Pakistan falls into the hands of the militants, that will be a worry."

And, with Pakistan's nuclear capability, the stability of its government is of vital interest to the West, says Prof Campbell. "There is a scenario where there is popular opposition and there is a revolt with the fundamentalists taking power," he says. "Then we have a state with a fundamentalist government, that, for the first time, has a nuclear weapon. That is not a desirable outcome.

"The US planners have to allow for the unintended consequences which could make a massive military operation against Afghanistan seriously counter-productive. Not only could it kill thousands of innocent civilians but it could cause problems in Pakistan.

"It is an anxious time and the consequences are more serious than they were in the Gulf War. At least Saddam Hussein was not completely irrational and didn't want to be pulverised into the ground. He operates with the same sort of logic that other states and leaders do. Bin Laden, and the people who follow him, don't. Provoke that, and that will change all our daily lives for some time to come."