US officials based in Pakistan were sounding out senior members of her opposition Pakistan People's party about a possible successor. They were also in contact with members of the other main opposition party, the Pakistan Muslim League, led by Nawaz Sharif, even though the US had previously opposed his return to Pakistan because of links between his party and Islamist extremists.
President George Bush called for the election to go ahead, though he avoided mention of whether Pakistan should stick to the January 8 timetable. An announcement on whether to delay the election has been left until the end of the three days of mourning.
Asked whether the US was confident that Pakistan could stage an election in January, the US state department spokesman, Tom Casey, said: "Well, we're going to see what happens."
The assassination of Bhutto has thrown into disarray Bush administration hopes of establishing a degree of security in Pakistan. Since 9/11, Bush has relied on the military-run government of President Pervez Musharraf as an ally in the fight against the Taliban and al-Qaida. With Musharraf's loss of popularity, the administration placed its hopes on a return to democracy and the emergence of a Musharraf-Bhutto coalition.
US intelligence analysts warned that al-Qaida, which has a hold in Pakistan's tribal areas - where the US believes Osama bin Laden is hiding - and in cities such as Karachi would be strengthened by the chaos in the aftermath of the assassination.
John McLaughlin, former acting director of the CIA, predicted that the chaos would last for weeks at least and that the capacity of Pakistan's authorities to deal with al-Qaida during that time would be diminished.
The Bush administration is worried that Pakistan could fall into the hands of radicals, undermining its battle against the Taliban and al-Qaida in Afghanistan and raising the spectre of nuclear weapons in the hands of Islamists hostile to the west.
The present head of the CIA, Robert Gates, warned last week that al-Qaida was focusing its efforts on Pakistan.
The US helped engineer the return of Bhutto to the US in October after eight years of self-imposed exile. It will be harder for US officials to persuade Musharraf, who has stood down as commander of the army to contest the election as a civilian, to enter into a coalition with Sharif, given the degree of enmity between them.
On Thursday Bush called on Pakistan "to honour Benazir Bhutto's memory by continuing with the democratic process for which she so bravely gave her life." But he made no mention of January 8.
Gordon Brown, who also spoke to Musharraf, echoed Bush's comments, calling on him to "stick to the course he has outlined to build democracy and stability in Pakistan". He did not specifically say elections should be held on January 8.
"This was a cowardly terrorist act designed to destabilise democratic elections. The international community is united in its outrage and determination that those who stoop to such tactics shall not prevail," he said. Nicolas Sarkozy, the French president, made a similar call for the election to go ahead.
Stephen Cohen, a specialist on India and Pakistan at the Brookings Institution, a non-profit policy studies thinktank, described Bhutto's death as a "blow to the idea of a liberal, moderate Pakistan" that made him fear for that country.
"Its further decay will affect all of its neighbours, Europe and the United States in unpredictable and unpleasant ways," he said.
Explainer: The nuclear arsenal
The Pentagon is working on contingency plans to prevent Pakistan's nuclear arsenal falling into the hands of Islamist radicals and insisted yesterday that the arsenal was safe in spite of the upheaval in the aftermath of the assassination of Benazir Bhutto.
Pentagon spokesman Colonel Gary Keck said: "At this time, we have no need for concern." However, the Bush administration is less confident about the future. The US administration has spent $100m (£50m) over the last six years on improving the security of Pakistan's nuclear programme.
One of the contingency plans would involve US special forces, working with Pakistan's military and intelligence services, to spirit away any weapons at imminent risk. But the US cannot be confident that the Pakistan military would cooperate at such a time.
In spite of US aid to help with security, the Pakistan government has remained suspicious of US intentions, fearing that it might plant devices capable of neutralising the weapons.
As a result, Pakistan has withheld information about the location of all its arsenal and other specifics. Pakistan's nuclear scientists and technicians go to the US for training.
Pakistan carried out its first nuclear test in 1998 and claims to have about 80 to 120 warheads. It has many decoys to confuse would-be thieves.
Pakistan's nuclear weapons are stored in bunkers in about half-a-dozen military bases and, to provide a degree of safety, the components are kept separately.
Guardian Unlimited © Guardian News and Media Limited 2007