WASHINGTON - President Bush held an emergency meeting of his top foreign-policy aides yesterday to discuss the deepening crisis in Pakistan as administration officials and others explored whether Thursday's assassination of opposition leader Benazir Bhutto marks the beginning of a new Islamic extremist offensive that could spread beyond Pakistan and undermine the U.S. war in neighboring Afghanistan.

U.S. officials fear that a renewed campaign by Islamic extremists aimed at the Pakistani government, and based along the border with Afghanistan, would complicate U.S. policy by effectively merging the war in Afghanistan with Pakistan's turbulence.

"The fates of Afghanistan and Pakistan are inextricably tied," said J. Alexander Thier, a former U.N. official in Afghanistan now at the U.S. Institute for Peace.

U.S. military officers and other defense experts say they do not anticipate an immediate impact on U.S. operations in Afghanistan. But they are concerned that continued instability eventually will spill over and intensify the fighting in Afghanistan, which has spiked in recent months as the Taliban has strengthened and expanded its operations.

Unrest in Pakistan and rising fuel prices have already boosted the cost of food in Afghanistan, making it more likely that Afghans will be lured by payments from the Taliban to participate in attacks, a U.S. Army officer in Afghanistan said.

In a secure videoconference yesterday linking officials in Washington, Islamabad and Crawford, Texas, Bush received briefings from CIA Director Michael Hayden and the U.S. ambassador to Pakistan, Anne Patterson, said National Security Council spokesman Gordon Johndroe. Bush then discussed Bhutto's assassination and U.S. efforts to stabilize Pakistan with his top foreign-policy advisers.

Secretary of State Condoleezza Rice, after signing a condolence book for Bhutto at the Pakistani Embassy in Washington, said the United States was in contact with "all" of the parties in Pakistan and stressed that the Jan. 8 elections should not be postponed. "Obviously, it's just very important that the democratic process go forward," she told reporters.