WASHINGTON — The Bush administration scrambled Thursday to deal with the assassination of former Pakistani Prime Minister Benazir Bhutto's after having investing significant diplomatic capital in promoting reconciliation between her and President Pervez Musharraf.

President Bush, speaking briefly to reporters at his ranch in Crawford, Texas, urged that her murder not derail nascent efforts to restore democratic rule ahead of parliamentary elections set for next month. And he demanded that those responsible for the killing be brought to justice.

"The United States strongly condemns this cowardly act by murderous extremists who are trying to undermine Pakistan's democracy," said Bush, who looked tense and took no questions. He expressed his deepest condolences to Bhutto's family and to the families of others slain in the attack and to all the people of Pakistan.

Secretary of State Condoleezza Rice echoed those comments, calling on Pakistanis to remain calm.

"The deadly results of this attack will no doubt test the will and patience of the people of Pakistan," Rice said in a statement. "We urge the Pakistani people, political leaders, and civil society to maintain calm and to work together to build a more moderate, peaceful, and democratic future."

Bush's appearance came as U.S. officials struggled with the implications of the assassination on relations with a nuclear-armed country that has received billions of dollars in American financial assistance and is a key ally in the war on terrorism. White House spokesman Scott Stanzel said Bush spoke briefly by phone with Musharraf but he had no details.

Bhutto was mortally wounded Thursday in a suicide attack that also killed at least 20 others at a campaign rally in Rawalpindi. She served twice as Pakistan's prime minister between 1988 and 1996. She had returned to Pakistan from an eight-year exile Oct. 18. Her homecoming parade in Karachi was also targeted by a suicide attacker, killing more than 140 people.

Stanzel said it was too soon to say who was responsible.

"I'm aware that al-Qaida may have claimed responsibility," Stanzel said. "I'm aware of news reports of that. But I don't have any specifics for you on that." He did say, "Whoever perpetrated this attack is an enemy of democracy and has used a tactic that al-Qaida is very familiar with, and that is suicide bombing and the taking of innocent life to try to disrupt the democratic process."