Bush telephoned Pakistani President Pervez Musharraf to discuss the situation, the White House said, while a spokesman appealed for calm in Pakistan amid deadly riots following Bhutto's murder.

"The US strongly condemns this cowardly act by murderous extremists who are trying to undermine Pakistan's democracy," Bush told reporters hours after Bhutto was killed in the Pakistani city of Rawalpindi.

"Those who committed this crime must be brought to justice," he said near his ranch in Texas where he is spending the end-of-year holidays.

"We stand with the people of Pakistan in that struggle against the forces of terror and extremism."

Bush called on Pakistanis to stay on the path of democracy after Musharraf ended six weeks of emergency rule ahead of elections that are due to be held in the country on January 8.

"We urge them to honor Benazir Bhutto's memory by continuing with a democratic process," he said.

Bush called Musharraf at 1815 GMT, spokesman Scott Stanzel said, giving no details on the conversation except to say it was "brief."

The suicide attack at a rally for Bhutto left at least 20 people dead and raised new questions about Pakistan's stability under Musharraf ahead of next month's vote.

At least four people were killed later Thursday as angry mobs took to the streets of Pakistani cities, torching scores of vehicles and buildings.

"We would urge calm, and there is a risk of, after an assassination like this of a political leader, there is a risk of people turning to violence to express their anger," Bush spokesman Stanzel told reporters.

He said it was too early to assign blame for Bhutto's murder but said its perpetrator "has used a tactic which Al-Qaeda is very familiar with, and that is suicide bombing and the taking of innocent lives to try to disrupt a democratic process."

Elsewhere, US officials voiced concern over the implications of Bhutto's assassination.

"We obviously condemn the attack that shows that there are people out there who are trying to disrupt the building of democracy in Pakistan," said Deputy State Department spokesman Tom Casey.

Arizona Senator John McCain, a Republican presidential candidate, said Bhutto's death "underscores yet again the grave dangers we face in the world today."

"Given Pakistan's strategic location, the international terrorist groups that operate from its soil, and its nuclear arsenal, the future of that country has deep implications for the security of the United States and its allies," he said.

Long-shot Democratic candidate Senator Joseph Biden who chairs the Senate Foreign Relations committee had recently written to Musharraf, asking for a greater protection for the opposition leader.

"This is a terrible day," Biden said. "I am convinced Ms Bhutto would have won free and fair elections next week."

Senator Chris Dodd, another Democratic presidential candidate, said the killing underscored the need for stability in Pakistan.

"At this critical time we must do everything in our power to help Pakistan continue the path toward democracy and full elections. Our first priority must be to ensure stability in this critical nuclear state," he said.

Washington has been taking a cautious line toward its key ally in the war on terror, particularly after Musharraf imposed emergency rule from November 3 to December 15 and cracked down on his opposition.

The moves prompted both a review of US aid to Pakistan and a broader debate on its status as an ally, but Bush's administration stayed upbeat toward Musharraf.

US officials have insisted on the need to ensure free and fair elections next month but have shied away from speculating on what they would do if the polls turned out to be rigged in any way.

But the Bush administration was also facing a restive US Congress, which sought to slap restrictions on military aid to Pakistan, a nuclear power along with its subcontinent rival India.

Lawmakers moved to put limits on 300 million dollars of US military aid received each year by Pakistan.

In the past, Pakistan enjoyed free reign to use the funds, but under a catch-all budget bill passed by Congress last week, 250 million dollars of those funds now are to be used strictly for counter-terrorism operations.

Congress stipulated that the remaining 50 million dollars were to be withheld until the Bush administration demonstrates that Pakistan is making clear moves toward democracy.

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