"There are obstacles in the face of forming this coalition, because al-Sistani does not support it. So we will work to strengthen the [Shia] alliance," said Hassan al-Sunnaid, of the Dawa party of Nuri al-Maliki, Iraq's prime minister.
Ali al-Adib, also a Dawa member, said al-Sistani "does not support such blocs because they will break [Shia] unity".
An official close to al-Sistani said the cleric "will not bless nor support any new bloc or front. He only supports the unity of the Shias".
After meeting al-Sistani, the Shia politicians visited al-Sadr, the Mahdi Army chief. Al-Sadr had pulled his supporters out of Iraq's coalition government last month after al-Maliki met George Bush, the US president, for crisis talks.
The Shia leader has now agreed to allow his supporters to rejoin the government, officials close to him have said. However, no agreement was reached on the timetable for withdrawal of US-led troops from Iraq.
"They agreed to all our demands, except scheduling a withdrawal for the occupier. This demand will be discussed during the next meeting," Abu Firas al-Mutairi, a political official from Sadr's movement, said.
"Our meeting with Moqtada al-Sadr was successful and fruitful. There were common points of understanding between us, and we assured the unity of the [United Iraqi] Alliance"
Khaled al-Attiya, an independent and deputy speaker of parliament
"Our meeting with Moqtada al-Sadr was successful and fruitful. There were common points of understanding between us, and we assured the unity of the [United Iraqi] Alliance," said Khaled al-Attiya, an independent and deputy speaker of parliament.
"[Al-Sadr] will give his final decision to rejoin the government and parliament after Eid al-Adha," he said, referring to the Islamic calendar's most important holiday, which Iraq's Shias will celebrate from December 31 to January 4.
Also on Saturday, Robert Gates, the new US defence secretary, met Bush to offer advice on transforming US policy in Iraq.
The US president is considering whether to quickly send more US troops to Iraq to control the violence. There are currently 140,000 American troops in Iraq.
Gates has declined to say whether he plans to recommend a short-term increase in US troop levels, but said he believes there is "a broad strategic agreement between the Iraqi military and Iraqi government and our military".
Meanwhile, in continuing violence, six civilians were killed on Saturday when a mortar shell hit a house in Baquba, and an Iraqi military intelligence officer was killed in a drive-by shooting south of Baghdad, according to the police.
Clashes between Iraqi security forces and Shia fighters loyal to al-Sadr left six people dead and wounded 50 others, including 13 civilians, in the southern Iraqi city of Samawa, police sources said.
Three US marines were also killed in Anbar province, a Sunni area in the east of Iraq, according to the US military.
On Friday, the US military announced five more American troop deaths, making December the second deadliest month for US servicemen in 2006.