ISLAMABAD, Pakistan - Former Pakistani leader Nawaz Sharif said Thursday he would return home from exile on Sept. 10 to challenge President Gen. Pervez Musharraf's plans to extend his rule.

The announcement came a day after Benazir Bhutto, another exiled former premier and Sharif's rival, said she was progressing toward an agreement with Musharraf that could see them share power.

Bhutto claimed Musharraf had agreed to step down as head of the army, ending military rule eight years after the general ousted Sharif in a bloodless coup. However, a Pakistani government spokesman said Thursday that although Musharraf was discussing the issue, no decision had been made.

Sharif immediately challenged any agreement between Musharraf and Bhutto.

"If a deal is struck between Benazir Bhutto and Mr. Musharraf, it will not serve the cause of restoring democracy to Pakistan,'' Sharif said. "It will only be strengthening the hands of a dictator, and we want to get rid of the dictatorship in Pakistan.''

Pakistan's Supreme Court ruled last week that the conservative, secularist Sharif, who has been in exile since 2000, could return to Pakistan.

However, government officials have said that Sharif, who insists Musharraf must be removed from both the government and the army, could be re-arrested upon reaching Pakistani soil on charges dating to the 1999 coup.

Musharraf had vowed to prevent both Bhutto and Sharif from entering Pakistan again, blaming them for corruption and economic problems that nearly bankrupted the country in the 1990s, when each served as prime minister.

With Musharraf's support eroding, he has edged toward an alliance with Bhutto and her moderate Pakistan People's party so he can be re-elected as a strong civilian president backed by a friendly parliament.

The prospect of Sharif making a tumultuous return has added to the urgency of an accommodation between Musharraf and Bhutto, who share a relatively liberal, pro-western outlook and stress the need to prevent the political crisis from destabilizing the nuclear-armed nation.

Under the proposed pact, the government would drop corruption cases against Bhutto to allow her to return home and abolish regulations that prevent her from serving a third term as prime minister. But Sharif's return could trigger political turmoil and upset their calculations.

Bhutto told The Associated Press on Wednesday that she was "very pleased that Gen. Musharraf has taken the decision to listen to the people of Pakistan by taking the decision to take off the uniform'' as part of their agreement.

"I expect that he will step down (as army chief) before the presidential elections, but that is for the president to say,'' she said.

Asked about Bhutto's assertion, government spokesman Mohammed Ali Durrani said: "No decision has been made.''

Durrani stopped short of denying that Musharraf was prepared to step down as army chief as part of a possible agreement.

"So far as dialogue is concerned, there can be discussion on everything,'' he said, but added that the issue of Musharraf's military uniform "will be decided by the president himself. If anyone else says something about this, it is just their own opinion.''

Asked if Musharraf will run in the upcoming election as military chief, Durrani replied: "He has a very clear stand on this that he will decide it himself and in the light of the constitution and law.''

Musharraf's office said earlier he would not succumb to "pressure or ultimatum'' in deciding whether to quit the army.

At stake is a pact that would protect Musharraf's troubled re-election bid from looming legal challenges and public disenchantment with military rule. In return, Musharraf is expected to give up his role as army chief and let Bhutto return from exile in London to contest year-end parliamentary elections.

Musharraf has seen his authority erode since March, when he tried unsuccessfully to remove the Supreme Court's top judge. The move triggered protests that grew into a broad pro-democracy campaign.

The court reinstated the judge in July, raising expectations that it will uphold legal challenges to Musharraf's re-election plan.

Officials say the pact with Bhutto would include constitutional amendments to forestall those challenges.

Musharraf has insisted that the constitution allows him to be army chief until the end of 2007 but has never made clear when -- or if -- he will step down.

However, Bhutto and other opposition leaders argue the constitution obliges him to give up that post before he asks legislators for a fresh presidential mandate in September or October.