America's top man in Iraq has urged the US-appointed Iraqi Governing Council to stick to the handover deal, despite renewed calls for early elections.
Ayatollah Sistani is the most senior Shia cleric in Iraq
Paul Bremer said "it is important to implement" the deal between the IGC and the US-led coalition to transfer power to a provisional government in June.
Earlier, a top Iraqi Shia cleric said an interim cabinet must be elected not appointed, as the plan envisages.
Grand Ayatollah Ali Sistani said "each Iraqi must have the right to vote".
The 15 November agreement calls for forming by June a caretaker government whose members would be selected by a transitional assembly.
The assembly's members will have been nominated by the IGC and local government officials across the country.
Under the deal, elections are not due until 2005.
"We think it is important to implement the... agreement, which was agreed by the Governing Council and has been submitted to the United Nations as the best way forward for the return of sovereignty to the Iraqi people," Mr Bremer said in Baghdad.
He added that the deal would also "provide for elections in about a year now to a constituent assembly".
At the same time, Mr Bremer said he wanted to leave the discussions on the issue to "the Governing Council and the Ayatollah (Sistani)".
An IGC spokesman on Monday also rejected Ayatollah Sistani's demands, saying that Iraq's war-torn infrastructure was simply not ready for elections.
"It is not possible to conduct a proper census... with the country functioning as it is at the moment," the spokesman, Hamid al-Kifai, was quoted as saying by the AFP news agency.
"At the moment we are proceeding with the agreement we have reached with the coalition and hopefully the time line we have agreed to will be adhered to," Mr Kifai added.
In remarks published on Monday, Ayatollah Sistani repeated his earlier warnings that without a legitimising vote, the proposed interim government would not have the right to govern Iraq in the coming transitional period.
"We want free elections and not appointments," Ayatollah Sistani wrote in the Iraqi daily Al-Zaman newspaper.
"Each Iraqi must have the right to vote for whomever they trust and want to represent them in the future assembly," his statement said.
"They want to disfigure the meaning of democracy and freedom that our people enjoy," the statement added.
Ayatollah Sistani had previously said he would accept a decision by a UN team whether it was feasible to hold elections in Iraq's current highly volatile atmosphere.
Ayatollah Sistani is the prime marja, or spiritual reference, for Shias everywhere.
He is one of only five living grand ayatollahs and the most senior Shia cleric in Iraq.