BAGHDAD (REUTERS) - A bloc comprising Iraq's biggest Shiite parties is close to nominating a prime minister, the deputy speaker of parliament said on Monday, directly challenging Nuri al-Maliki who has refused to give up his bid for a third term.

Haider al-Abadi's comments in a tweet came after police sources said special forces and Shiite militias loyal to Maliki had been deployed in strategic areas of Baghdad after he made a defiant speech on television suggesting he would not cave in to pressure to drop his bid for another term.

Abadi is one of the people that has been mentioned as a possible successor to Maliki. In his tweet, Abadi said government forces were moving around the capital in anticipation of security breaches.

Maliki accused Iraq's Kurdish President Fouad Masoum of violating the constitution by missing a deadline for him to ask the biggest political bloc to nominate a prime minister and form a government.

"I will submit... an official complaint to the federal court against the president of the Republic for committing a clear constitutional violation for the sake of political calculations," Maliki said in the televised speech.

Serving in a caretaker capacity since an inconclusive election in April, Maliki has defied calls by Sunnis, Kurds, some fellow Shiites, regional power broker Iran and Iraq's top cleric to step aside for a less polarising figure. Critics accuse Maliki of pursuing a sectarian agenda which has sidelined Sunnis and prompted some of them to support Islamic State militants, whose latest sweep through northern Iraq has alarmed the Baghdad government and its Western allies.

Maliki under fire

Washington seems to be losing patience with Maliki, who has placed Shiite political loyalists in key positions in the army and military and drawn comparisons with executed former dictator Saddam Hussein, the man he plotted against from exile for years.

Deputy State Department spokesman Marie Harf reaffirmed Washington's support for a "process to select a prime minister who can represent the aspirations of the Iraqi people by building a national consensus and governing in an inclusive manner".

"We reject any effort to achieve outcomes through coercion or manipulation of the constitutional or judicial process," Harf said in a statement, adding that the United States "fully supports" Masoum in his role as the guarantor of Iraq's constitution.

US President Barack Obama has urged Iraqi politicians to form a more inclusive government that can counter the growing threat from the Islamic State. But Maliki, an unknown when he first took office in 2006 with help from the United States, is digging in.

"Now we can see unprecedented deployment of army commandos and special elite forces deployed in Baghdad, especially sensitive areas close to the green zone and the entrances of the capital," one of the police sources said. "These forces are now taking full responsibility of securing these areas of the capital."

Iraq's Interior Ministry has told police to be on high alert in connection with Maliki's speech, a police official told Reuters.

More US air strikes

The Islamic State has capitalised on the political deadlock and sectarian tensions, making fresh gains after arriving in the north of the country in June from Syria. The group, which sees Iraq's majority Shiites as infidels who deserve to be killed, has ruthlessly moved through one town after another, using tanks and heavy weapons it seized from soldiers who have fled in their thousands.

Islamic militants have killed hundreds of Iraq's minority Yazidis, burying some alive and taking women as slaves, an Iraqi government minister said on Sunday, as US warplanes again bombed the insurgents. Human rights minister Mohammed Shia al-Sudani accused the Sunni Muslim militants - who have ordered the community they regard as "devil worshippers" to convert to Islam or die - of celebrating what he called a "a vicious atrocity".

No independent confirmation was available of the killings of hundreds of Yazidis. The bloodshed could increase pressure on Western powers to do more to help tens of thousands of people, including many from religious and ethnic minorities, who have fled the offensive.

The US Central Command said drones and jet aircraft had hit islamic armed trucks and mortar positions near Arbil, the capital of the autonomous Kurdish region which had been relatively stable throughout the past decade until insurgents swept across northwestern Iraq this summer. That marked a third successive day of US air strikes, and Central Command said that they were aimed at protecting Kurdish peshmerga forces as they face off against the militants near Arbil, the site of a US consulate and a US-Iraqi joint military operations centre.

The Islamists' advance in the past week has forced tens of thousands to flee, threatened Arbil and provoked the first US attacks since Washington withdrew troops from Iraq in late 2011, nearly nine years after invading to oust Saddam Hussein.

Women held as slaves

Consolidating a territorial grip that includes tracts of Syrian desert and stretches toward Baghdad, the local and foreign fighters have swept into areas where non-Sunni groups live. While they persecute non-believers in their path, that does not seem to be the main motive for their latest push. The group wants to establish religious rule in a caliphate straddling Syria and Iraq and has tapped into widespread anger among Iraq's Sunnis at a democratic system dominated by the Shi'ite Muslim majority following the US invasion of 2003.

Sudani said: "The terrorist Islamic State has also taken at least 300 Yazidi women as slaves and locked some of them inside a police station in Sinjar and transferred others to the town of Tal Afar. We are afraid they will take them outside the country.

"In some of the images we have obtained there are lines of dead Yazidis who have been shot in the head while the Islamic State fighters cheer and wave their weapons over the corpses," he added. "This is a vicious atrocity."

Iraqis have slipped back into sectarian bloodshed not seen since 2006-2007 - the peak of a civil war. Nearly every day police report kidnappings, bombings and execution-style killings. The Sunni militants routed Kurds in their latest advance with tanks, artillery, mortars and vehicles seized from fleeing Iraqi troops. The militants are now just 30 minutes' drive from Arbil.

In their latest sweep through the north, the Sunni insurgents seized a fifth oil field, several more villages and the biggest dam in Iraq - which could give them the ability to flood cities or cut off water and power supplies - hoisting their black flags along the way. After spending more than US$2 trillion (S$2.5 trillion) on its war in Iraq and losing thousands of soldiers, the United States must now find ways to tackle a group that is even more hardline than al-Qaeda and has threatened to march on Baghdad.