Top presidential candidates said Sunday the United States was stuck for now with Pakistan's unpopular President Pervez Musharraf at a time of boiling unrest in the nuclear-armed US ally.

But Democratic and Republican runners said the US administration should still be pressing Musharraf for tougher anti-terror action, and free elections, following last week's assassination of opposition leader Benazir Bhutto.

"I'm not calling for him to step down," Democrat Hillary Clinton told ABC television, as Benazir's murder revived debate about candidates' foreign-policy credentials just before Iowa kicks off the 2008 White House race on Thursday.

"I'm calling for him, number one, to agree with an independent investigation of Benazir Bhutto's death. I am calling on him to hold free and fair elections with independent monitors," the senator and former first lady said.

Clinton's interview was taped before Benazir's 19-year-old son Bilawal was chosen Sunday to assume the mantle of her party, which said it would take part in elections on January 8 despite unrest triggered by the assassination.

Clinton stressed: "This is an opportunity for President Musharraf to step up and actually fulfil many of the words and promises that he's made to me and to many others over the course of a number of years."

Democratic challenger Barack Obama denied making political hay out of Benazir's murder, after arguing that Clinton’s vote in 2002 for the Iraq war had stoked unrest in Pakistan by distracting US attention from Afghanistan.

"What I do believe is that if we are going to take seriously the problem of Islamic terrorism, and the stability of Pakistan, then we have to look at it in a wider context, " he said on NBC.

Obama said the US administration's hefty financial support for its "war on terror" ally should not

translate into a "blank check" for Musharraf, and urged the Pakistani leader to earn greater legitimacy through open elections.

Democratic contender John Edwards told the CBS network he had spoken to Musharraf after Benazir's killing and had insisted that "democratisation in Pakistan must continue."

"He assured me that it would. That has to be taken with a huge grain of salt, given his history," Edwards said.

Republican hopeful John McCain, a Vietnam war hero and foreign-policy veteran, said Musharraf was "important" to Pakistan's future and echoed Clinton in asking "who will take his place?"

Senator McCain said he was "disappointed" that under Musharraf, parts of northwest Pakistan had become "a safe haven for Taliban."

But die president can still "play a key and pivotal role" in moving Pakistan back towards democracy, he said on ABC.