In an interview on Friday, Tariq Azim, deputy information minister, also drew a clear distinction between the rights of Sharif and Bhutto to return to Pakistan.
Azim said: "Nawaz Sharif's case was different. He went back to Saudi Arabia because of an undertaking he had with the Saudi government. She [Bhutto] was always allowed to come back."
Asked about pending corruption cases against Bhutto, he said: "It's for the law to take its own course. Everybody has to face cases against them and the same applies to her."
Azim said talks with Bhutto were being delayed over her desire for the corruption cases to be closed, for a constitutional amendment to let her seek a third term as prime minister, and over the president's re-election.
He said: "The talks are continuing but not at the same pace we might have wished. It's in the national interest for a resolution between political leaders to be reached.
"But it should be in the national interest, not in the personal interest of anyone."
Bhutto has led her party from exile since leaving Pakistan in 1999 over the corruption allegations.
She risks a backlash among the public and her party if she strikes an agreement with Musharaff, who ousted Sharif in a 1999 coup.
On Friday, Sharif's party urged her not to reach terms with Musharraf.
Sadiq ul-Farooq, a senior figure in the party, said: "We welcome her coming back, but let me say that it will be an insult to democracy if she agrees to share power with a man who ousted the elected government of Nawaz Sharif and has caused irreparable damage to democratic institutions."
Musharraf, an important ally of the US, has been trying for months to reach an agreement with Bhutto that would overcome legal obstacles to him seeking a new five-year term.
Wife as backup?
With less than five weeks before the presidential election, Bhutto's party says time is running out, though with Sharif out of the way, Musharraf may be in a stronger position to dictate terms.
Azim said the schedule for the presidential vote would be announced in the next three or four days. General elections are due by January.
Azim confirmed reports that Chaudhry Shujaat Hussain, the chief of the ruling party, had suggested that Musharraf's wife, Sebha, could be a backup candidate for the presidency if Musharraf was ruled ineligible to run.
Azim defended the notion as "traditional" in Pakistani politics but said "I don't know if it would be acceptable to the president or his wife."