Washington ? The assassination of former Pakistani Prime Minister Benazir Bhutto has dealt a severe blow to U.S. efforts to restore stability and democracy in a turbulent, nuclear-armed Islamic nation that has been a critical ally in the war on terror.

While not entirely dependent on Bhutto, recent Bush administration policy on Pakistan had focused heavily on promoting reconciliation between the secular opposition leader who has been dogged by corruption allegations and Pakistan’s increasingly unpopular president, Pervez Musharraf, ahead of parliamentary elections set for January.

In Washington and Islamabad, U.S. diplomats urged that Jan. 8 elections should not be postponed and strongly advised against a reimposition of emergency rule that Musharraf lifted just weeks ago.

FBI and Homeland Security officials sent a bulletin late Thursday to U.S. law enforcement agencies citing Islamist Web sites as saying al-Qaida had claimed responsibility for the attack and that the group’s No. 2 leader, Ayman al-Zawahri, had planned it.

The intelligence community is using all of its resources to determine who was behind the Bhutto assassination, Director of National Intelligence spokesman Ross Feinstein said. But he added, “We’re in no position right now to confirm who may have been responsible for the attack.”

FBI spokesman Richard Kolko said the bureau was reviewing the al-Qaida claims for any intelligence value. “The validity of those claims are undetermined,” he said.

The U.S. has poured billions of dollars in financial assistance into Pakistan since Sept. 11, 2001, when Musharraf made a calculated decision to align his government with Washington in going after al-Qaida and the Taliban in neighboring Afghanistan. That move is blamed for several unsuccessful assassination attempts on him.

But it was not immediately clear, however, what, if any, influence Washington might have or whether Bhutto’s death would drive the United States into a deeper embrace of Musharraf, whom some believe offers the best chance for Pakistani stability despite his democratic shortcomings.

“This latest tragedy is likely to reinforce beliefs that Pakistan is a dangerous, messy place and potentially very unstable and fragile and that they need to cling to Musharraf even more than they did in the past,” said Daniel Markey, who left the State Department this year and is now a senior fellow at the private Council on Foreign Relations. “The weight of the administration is still convinced that Musharraf is a helpful rather than a harmful figure.”

Amid the political chaos and uncertainty roiling the country in the wake of Bhutto’s slaying, U.S. officials scrambled Thursday to understand the implications for the massive aid and counterterrorism programs that have been criticized by lawmakers, especially as al-Qaida and Taliban extremists appear resurgent along the Pakistan-Afghan border.

Underscoring the concerns, a grim President Bush interrupted his vacation to personally condemn Bhutto’s murder, demanding that those responsible be brought to justice and calling on Pakistanis to continue to press for democracy.

“We urge them to honor Benazir Bhutto’s memory by continuing with the democratic process for which she so bravely gave her life,” Bush told reporters at his Texas ranch, before speaking briefly to Musharraf by phone.

Secretary of State Condoleezza Rice said Bhutto’s assassination would “no doubt test the will and patience of the people of Pakistan” but called on the Pakistani people in a statement “to work together to build a more moderate, peaceful and democratic future.”

Yet such calls could fall on deaf ears, experts said.

“The United States does not have a great deal of leverage where Pakistan is concerned,” said Wendy Sherman, who served as counselor to former Secretary of State Madeleine Albright. “And at the end of the day, the decisions are going to be made by the Pakistani people and by the leadership of Pakistan and not by the United States.”