Pakistan's opposition election winners were due to step up efforts to forge a coalition on Friday, raising the prospect of a government intent on forcing U.S. ally President Pervez Musharraf from power.

Leaders of the two parties that won the election, the Pakistan People's Party (PPP) and the Pakistan Muslim League (Nawaz), or PML (N), vowed on Thursday to work together to form a new government but said they still had details to work out.

The main party that backs the unpopular Musharraf was dealt a stunning defeat in Monday's general elections leaving the president, who has been one of Washington's top Muslim allies against al Qaeda, vulnerable to a hostile parliament.

Nawaz Sharif, the prime minister Musharraf overthrew in 1999 and whose PML (N) came second in the vote, has demanded the unpopular president steps down.

But since the election, Musharraf has said he was not ready to resign.

U.S. President George W. Bush's administration has urged the next government to work with Musharraf and says Washington needs Pakistan -- which borders Afghanistan where U.S. and NATO forces are fighting Islamist militants -- as an ally.

Sharif met Asif Ali Zardari, Bhutto's widower and leader of her PPP since her murder on December 27, in Islamabad on Thursday evening for their first face-to-face talks since the election.

Sharif later told a news conference the two parties would work together to form a government. Zardari, whose party won the most seats in the election, said he wanted a broad government but one excluding the main party that backs Musharraf.