The warning came amid mounting pressure by Kurdish, Sunni and secular political groups for Shiites to ditch the outgoing prime minister, who is considered too partisan and unable to control the security situation.

"We hope our brothers in the Alliance will look objectively at our reservations concerning Jaafari," Thafer al-Ami, a spokesman for the National Concord Front, the main Sunni-based alliance, told AFP.

"We think that in the end they'll understand our point of view and will be broad-minded enough, otherwise Jaafari's chances in parliament will be slim."

The leading political party gets to select the prime minister, but he must also win a vote of confidence in parliament once he has formed his cabinet.

The Shiite-based alliance won 128 of parliament's 275 seats in the December general elections, less than the absolute majority, and will have to form an alliance with other factions in order to govern.

Political leaders, backed by the United States, have called for a broad-based "national unity" government involving the minority Sunnis and Kurds to help end violence in the country.

"It's in nobody's interest to stir up a political crisis for partisan or personal reasons," Ami said.

"We only ask for one thing -- that the Alliance choose another candidate. We respect its political weighting, but it must also take national unity into account," he added.

On Thursday the Kurdish, Sunni and secular political groups wrote to the Shiite Alliance to reject its selection of Jaafari to lead the next cabinet.

"We are waiting for an official, written reply from the Shiite list and then we will see, but the key point is that even half the Shiite members of their alliance do not want him," Mahmud Othman, a senior Kurdish parliamentarian told AFP.

"If the Shiites do not change him, he will find it difficult to form the next cabinet. Will he form it in the sky?," Othman asked.

Last month, Jaafari was re-selected for the post of premier by the Shiite Alliance, but only by one vote.

Critics say Jaafari's lack of charisma and support for radical Shiites in the outgoing government has undermined his political credibility at a time when the government faces increased pressure because of sectarian violence triggered by the bombing last month of a revered Shiite shrine.