The Iraqi officials say the army has thwarted an attack by militants on the country's largest oil refinery in Baiji, Iraq. (Reuters)

Iraq’s leading Shiite politicians met Thursday to discuss their nomination for prime minister, divided over who should steer the country as an al-Qaeda-linked insurgency threatens the future of the state.

Prime Minister Nouri al-Maliki is trying to shore up his fractured support ahead of the first session of Iraq’s new parliament next week. Some factions of both Sunnis and Shiites have called for him to step aside.

As Shiite leaders wrangled over their nomination choices, the urgency of ­efforts to reach a political consensus was underscored by a day of widespread violence.

Clashes engulfed the northern town of Tikrit, where Iraq’s military moved onto the offensive, launching a commando assault and claiming to have seized the university.

Meanwhile, Christian villagers were forced to flee attacks by the Islamic State of Iraq and Syria (ISIS) in the north, and violence south of Baghdad raised fears that Sunni militants could surround the capital.

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“The security breaches in the country mean we cannot waste time,” said Hakim al-Zamili, a parliamentarian from Shiite cleric Moqtada al-Sadr’s party, which opposes Maliki. He said Iran is pushing for the Shiites to unite, but “there is still no clear vision on a single nominee for prime minister.”

Maliki’s Shiite-led bloc won 92 of the parliament’s 328 seats in April elections, but it needs a majority to form a government. The violence that has swept the country since Iraq went to the polls has heightened questions over whether he can do that.

Iraq’s presidency announced Thursday that the first session of the country’s newly elected parliament will be held Tuesday, starting the difficult process of forming a new government.

Politicians aligned with Sadr remain vocally opposed to Maliki’s premiership, saying that if the prime minister’s bloc does not drop his candidacy, they will put forward their own. Another Shiite party, Ammar al-Hakim’s Islamic Supreme Council of Iraq, has made no public comment.

Under Iraq’s constitution, the parliament’s first step is to name a speaker, with a 30-day window for a new president to be chosen, who then has 15 days to ask the largest political bloc to chose a prime minister.

The Shiite parties increasingly look as though they may not pursue the kind of consensus government the United States is advocating, said Michael Knights, an analyst at the Washington Institute.

“They could say, this is a whole new level of crisis, forget the Sunnis, we smash ISIS, the Kurds have gone their own way,” he said. “I detect increasing amounts of that kind of mentality.”

Momentum on the battlefield could rally more support for Maliki, said Hunain al-Qaddo, a member of parliament with Maliki’s bloc.

“I genuinely believe Maliki will be the new prime minister,” he said, claiming that Maliki had already secured a 120-seat coalition.

In some areas of Iraq, the armed forces did seem to be gaining momentum Thursday. Paratroopers dropped into Tikrit, the home town of former leader Saddam Hussein, after an aerial bombardment, state television and military officials said. There were unconfirmed reports that at least one helicopter was shot down Thursday.

Elsewhere, ISIS made more gains, with militants overrunning several villages near the Mansuriya gas field, according to a senior official in Diyala province.

The rebels battled toward the key facility about 30 miles east of Baqubah after three days of fighting there, said the official, who spoke on the condition of anonymity because he is not authorized to speak to the news media.

On the border of the semiautonomous Kurdish region, residents of the historic Christian town of Qaraqosh fled heavy militant shelling, according to al-Qaddo, who represents the area in parliament. Kurdish troops had been securing the area.

Meanwhile, less than 10 miles south of the capital, nine bodies were found dumped under a highway bridge early Thursday, according to a police officer and local residents. The discovery harked back to the brutal killings that marked Iraq’s worst years of sectarian violence.

The police officer said that Shiite militias had set up checkpoints around Mahmudiyah after a mortar-and-bomb attack by ISIS militants Wednesday. Requesting anonymity for security reasons, he claimed that militiamen had snatched Sunnis from cars.

The insurgents’ attack on Mahmudiyah has raised concerns that militants could cut off the capital by destabilizing the “Baghdad belt,” home to a large Sunni population. One security official, who spoke on the condition of anonymity because of the sensitivity of the subject, said there was evidence that insurgents were seeking to launch a strike on Hilla, about 50 miles south of the capital, to throw the south, which has been stable, into disarray.

“They are incubating in the Sunni belt around Baghdad,” said Zamili, the parliament member, who was accused of running Shiite death squads during Iraq’s sectarian war. “It’s possible they will try to strangle us, but military calculations are being made.”