Iraqi Prime Minister Nuri al-Maliki broadcast a joint appeal for national unity on Tuesday with bitter Sunni critics of his Shi'ite-led government - a move that may help him win US help against rampant Islamists threatening Baghdad.

Just hours after Maliki's Shi'ite allies had angrily vowed to boycott any cooperation with the biggest Sunni party and his government had accused Sunni neighbor Saudi Arabia of backing "genocide", the premier's visibly uncomfortable televised appearance may reflect U.S. impatience with its Baghdad protege.

In a rerun of previous failed efforts at bridging sectarian and ethnic divisions, Shi'ite, Sunni and Kurdish leaders met behind closed doors and then stood frostily before cameras as Maliki's Shi'ite predecessor Ibrahim al-Jaafari read a statement denouncing "terrorist powers" and supporting Iraqi sovereignty.

U.S. President Barack Obama is considering military options to push back al Qaeda splinter group the Islamic State of Iraq and the Levant (ISIL), which has swept the Sunni north of the country over the past week as the Shi'ite-led army has crumbled.

But in return Washington want Maliki to do more to address the widespread sense of political exclusion among minority Sunnis which ISIL has exploited to win support among tribal leaders and former followers of ousted dictator Saddam Hussein.

"No terrorist powers represent any sect or religion," Jaafari said in the address, which included a broad promise of "reviewing the previous course" of Iraqi politics. Afterwards, most of the leaders, including Maliki and Usama al-Nujaifi, the leading Sunni present, walked away from each other in silence.

Earlier, Maliki's government accused Saudi Arabia, the main Sunni power, of backing ISIL - something Riyadh denies.

"We hold them responsible for supporting these groups financially and morally and for its outcome - which includes crimes that may qualify as genocide: the spilling of Iraqi blood, the destruction of Iraqi state institutions and historic and religious sites," a government statement said.

Maliki has blamed Saudi Arabia for supporting militants in the past, but the language was unprecedented. On Monday, Riyadh blamed sectarianism in Baghdad for fuelling the violence.

Maliki, who has been buoyed by a call by Iraq’s senior Shi’ite cleric for citizens to rally to the armed forces, dismissed four generals for abandoning the big northern city of Mosul a week ago and said they would face court martial.

BAQUBA BATTLE

Scores were killed on Tuesday in a battle for another provincial capital, close to Baghdad, and fighting shut Iraq's biggest refinery at Baiji, hitting fuel and power supplies.

Government forces said they repelled an overnight attempt by insurgents to seize Baquba, capital of Diyala. Some residents and officials said scores of prisoners from the local jail were killed. There were conflicting accounts of how they had died.

ISIL fighters who aim to build a Muslim caliphate across the Iraqi-Syrian frontier launched their revolt by seizing Mosul and swept through the Tigris valley towards Baghdad.

The fighters, who consider all Shi'ites to be heretics deserving death, pride themselves on their brutality and have boasted of massacring hundreds of troops who surrendered.

Western countries, including the United States, have urged Maliki to reach out to Sunnis to rebuild national unity as the only way of preventing the disintegration of Iraq.

"There is a real risk of further sectarian violence on a massive scale, within Iraq and beyond its borders," U.N. Secretary-General Ban Ki-moon said. "I have been urging Iraqi government leaders including Prime Minister al-Maliki to reach out for an inclusive dialogue and solution of this issue."

But the prime minister, in power for eight years and effective winner of a parliamentary election two months ago, seems instead to be relying more heavily than ever on his own sect, who form a majority long oppressed under Saddam.

Though the joint statement late on Tuesday said only those directly employed by the Iraqi state should bear arms, thousands of Shi'ite militiamen have been mobilized to defend Baghdad.

SCRAMBLING ALLIANCES

The sudden advance by Sunni insurgents has the potential to scramble alliances in the Middle East, with the United States and Iran both saying they could cooperate against a common enemy, all but unprecedented since the 1979 Iranian revolution.

Iran, the leading Shi'ite power, has close ties to Maliki and the Shi'ite parties that have won elections since U.S. forces toppled Saddam in 2003. Although both Washington and Tehran are allies of Baghdad, they have not cooperated in the past but diplomats discussed Iraq briefly on Monday in Vienna.

Obama, under fire at home by critics who say he did too little to shore up Iraq since withdrawing U.S. troops in 2011, is considering options including air strikes. He has sent a small number of extra marines to guard the U.S. embassy but has ruled out redeploying troops following their 2011 withdrawal.

Obama has invited Congressional leaders to talks at the White House on Wednesday as he considers his options in Iraq.

Iraqi officials confirmed that the Baiji refinery north of Baghdad had shut down, although they said government troops still held the vast compound. Foreign workers were evacuated by Iraqi government helicopters.

With the refinery shut, Iraq will have difficulty generating electricity and pumping water to sustain its cities in summer. There were already reports of queues for fuel in the north. One official with the Iraqi oil ministry said that northern and western Iraq would be hardest hit, while Baghdad would be less affected due to a refinery on its southern edge.

During the U.S. occupation, the refinery stayed open, and the threat to it shows how much more vulnerable Iraq is now to insurgents than it was before Washington pulled out troops.

Tens of thousands of Shi'ites have rallied at volunteer centers in recent days, answering a call by the top Shi'ite cleric to defend the nation. Many recruits are now in training.

But with the million-strong regular army abandoning ground despite being armed and trained by the United States at a cost of $25 billion, the government is increasingly relying for its own preservation on various Shi'ite militias, many of which operated during the death squad bloodletting of 2006-07.

According to one Shi'ite Islamist working in the government, well-trained organizations Asaib Ahl Haq, Khataeb Hezbollah and the Badr Organization are now being deployed alongside Iraqi military units as the main combat force.

CAPITAL BOMBED

Baghdad is on edge. Sunnis worry about convoys of civilian cars with bearded men in military uniform they assume are militiamen, while Shi’ites living in Sunni districts, are moving away, worried that a new round of civil war is unfolding.

Two attacks hit Shi'ite markets in Baghdad Tuesday, a suicide bomber and a car bomb. The two attacks left 18 dead and 52 wounded, according to medical and security sources.

The Sunni militants have moved at lightning speed, slicing through northern and central Iraq, capturing the towns of Hawija and Tikrit in the north before facing resistance in southern Salahaddin province, where there is a large Shi’ite population.

The battle lines are now formalizing, with the insurgents held at bay about an hour's drive north of Baghdad and just on the capital's outskirts to the west, beyond the airport.

Militants also attacked a town near the northern oil hub of Kirkuk that is inhabited by Shi'ite ethnic Turkmen. The fighting went back and forth and appeared a preview of the challenges the Kurds now facing having rolled into Kirkuk last week after the Iraqi army abandoned positions. A local official from said 5,000 Turkmen had fled. By nightfall, ethnic Kurdish fighters had cleared most of the town but militants still held some ground.

In a further sign of ethnic and sectarian polarization, Maliki allies have accused the Kurds of colluding with Sunnis to dislodge government forces in the north.

The mainly Turkmen city of Tal Afar, west of Mosul, fell to Sunni militants on Sunday, and the Iraqi military said it was sending reinforcements. The army said it killed a top militant named Abu Abdul Rahman al-Muhajir in clashes in Mosul.

But security officials seemed pessimistic. One warned: "There is no clear strategy for the Iraqi government to retake Mosul. And without the U.S. and international community support, the Iraqi government will never retake Mosul." - Reuters