BAGHDAD -- Prominent Shiite leaders pushed Thursday for the removal of Iraqi Prime Minister Nouri al-Maliki as parliament prepared to start work on putting together a new government, under intense U.S. pressure to rapidly form a united front against an unrelenting Sunni insurgent onslaught.

Increasingly, the Shiite al-Maliki's former allies believe he cannot lead an inclusive government that can draw minority Sunnis away from support for the fighters who have swept over a large swath of Iraq as they head toward the capital, Baghdad.

In a further sign of Iraq's unraveling along sectarian lines, a bombing on Thursday killed 12 people in a Shiite neighborhood of Baghdad that houses a revered shrine. Police also found the bullet-riddled bodies of eight Sunnis south of the capital.

Most crucially, though, backing for al-Maliki is weakening with his most important ally, neighboring Iran.

A senior Iranian general who met with Shiite politicians in Iraq during a 10-day visit this month returned home with a list of potential prime minister candidates for Iran's leadership to consider, several senior Iraqi Shiite politicians who have knowledge of the general's meetings told The Associated Press.

Iran's supreme leader, Ayatollah Ali Khamenei, wants al-Maliki to remain in his post, at least for now, the politicians said. However, Iran's moderate president, Hassan Rouhani, believes al-Maliki must go or else Iraq will fragment. Khamenei holds final say in all state matters in Iran, but the politicians expressed doubt he would insist on al-Maliki against overwhelming rejection of him by Iraq's Shiite parties.

The general, Ghasem Soleimani, is expected to return within days to inform Iraqi politicians of Tehran's favorite, the politicians said.

Iran has found in majority Shiite Iraq a convenient vehicle to extend its sphere of regional influence to the heart of the Middle East. Iran's leverage in Iraq also gives it a trump card against its Sunni rivals in the Gulf region, where powerhouse Saudi Arabia, for example, has traditionally viewed Tehran with suspicion.

The United States and its allies are pushing for the creation of a government that can draw support among Iraq's Sunni minority, which has been alienated by al-Maliki, seen as a fiercely partisan Shiite.

British Foreign Secretary William Hague, meeting with al-Maliki in Baghdad on Thursday, told a news conference that "we believe the urgent priority must be to form an inclusive government ... that can command the support of all Iraqis and work to stop terrorists and their terrible crimes."

Hague's trip follows a visit by U.S Secretary of State John Kerry, who earlier this week delivered a similar message.