The spiritual leader of Iraq's Shia majority called for a new, "effective" government Friday, increasing pressure on the country's prime minister a day after U.S. President Barack Obama challenged him to create a more inclusive leadership or risk a sectarian civil war.

Grand Ayatollah Ali al-Sistani's comments at Friday prayers contained thinly veiled criticism that Shia Prime Minister Nouri al-Maliki, in office since 2006, was to blame for the nation's crisis over the blitz by Sunni insurgents led by an al-Qaeda splinter group that seeks to create a new state spanning parts of Iraq and Syria and ruled by its strict interpretation of Islamic law.

Al-Sistani's remarks come as U.S. Secretary of State John Kerry is expected to travel to Iraq soon to press its government to share more power.

While al-Maliki's State of Law bloc won the most seats in parliament in the Iraq's April 30 election, his hopes for a third term are now in doubt with rivals challenging him from within the broader Shia alliance. In order to govern, his bloc must first form a coalition with other parties.

And with Iraq asking the U.S. for airstrikes to temper the militants' advance — especially as the insurgents were said to be preparing Friday for another assault on the country's biggest oil refinery — al-Maliki appears increasingly vulnerable.

"It is necessary for the winning political blocs to start a dialogue that yields an effective government that enjoys broad national support, avoids past mistakes and opens new horizons toward a better future for all Iraqis," al-Sisanti said in a message delivered by his representative Ahmed al-Safi in the Shia holy city of Karbala.

The Iranian-born al-Sistani, who is believed to be 86, lives in the Shia holy city of Najaf south of Baghdad. A recluse, he rarely ventures out of his home and does not give interviews. Iraq's majority Shias deeply revere him, and a call to arms he made last week prompted thousands of Shias to volunteer to fight against the Islamic State of Iraq and Syria, which was once part of al-Qaeda.

Al-Sistani's call to arms has given the fight against the Islamic State militants the feel of a religious war between Shias and Sunnis. His office in Najaf dismissed that charge, and al-Safi on Friday said: "The call for volunteers targeted Iraqis from all groups and sects. ... It did not have a sectarian basis and cannot be."

Al-Maliki has been seeking to place the blame for the chaos on the Islamic State and not his perceived exclusion of the Sunnis. However, questions persist on how much support, if any, the Islamic State enjoys among the Sunni population in areas it now controls.

'There can't be any trust' of ISIS

Ali Hatem al-Salman, a prominent tribal Sunni leader and a critic of al-Maliki, said Sunni tribesmen would eventually fight the extremist Islamic State.

Using the commonly used Arabic acronym for the Islamic State, he told The Associated Press on Thursday: "Daash themselves know that the tribes will push them out. ... There can't be any trust given to Daash."

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Members of the Iraqi Special Operations Forces take their positions during clashes with the al-Qaeda-linked Islamic State of Iraq and Syria. Sunni militants have taken over large swaths of Iraq in a lightning offensive. (Reuters)

Al-Maliki's Shia-led government long has faced criticism of discriminating against Iraq's Sunni and Kurdish populations. But it is his perceived marginalization of the once-dominant Sunnis that sparked recent violence reminiscent of Iraq's darkest years of sectarian warfare after the 2003 U.S.-led invasion.

Iraq's newly elected parliament must meet by June 30 to elect a speaker and a new president, who in turn will ask the leader of the largest bloc to form a new government within 15 days.

Shia politicians familiar with the secretive efforts to remove al-Maliki said two names mentioned as possible replacements are former vice president Adel Abdul-Mahdi, a French-educated economist who is also a Shia, and Ayad Allawi, a secular Shia who served as Iraq's first prime minister after Saddam Hussein's ouster.

Obama calls for 'inclusive agenda'

With Iraq in turmoil, al-Maliki's rivals have mounted a campaign to force him out of office, with some angling for support from Western backers and regional heavyweights. On Thursday, their effort received a boost from Obama, who said: "Only leaders that can govern with an inclusive agenda are going to be able to truly bring the Iraqi people together and help them through this crisis."

An "inclusive agenda" has not been high on the priorities of al-Maliki, however. Many of al-Maliki's former Kurdish and Shia allies have been clamouring to deny the prime minister a third term in office, charging that he has excluded them from a narrow decision-making circle of close confidants.

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Iraq's Shia prime minister, Nouri al-Maliki, embattled by the militant offensive sweeping his country's north, faces a growing campaign to force him out of office as insurgents press on with their campaign. (Ebrahim Noroozi/Associated Press)

Al-Maliki's efforts last year to crush protests by Sunnis complaining of discrimination under his Shiite-led government sparked a new wave of violence by militants, who took over the city of Fallujah in the western, Sunni-dominated province of Anbar and parts of the provincial capital, Ramadi.

Iraqi army and police forces battling them for months have been unable to take most areas back, and over the past week or so the militants have also taken over the cities of Mosul and Tikrit.

Less than three years after Obama heralded the end of America's war in Iraq, he said Thursday he was dispatching up to 300 military advisers to help quell the insurgency. They would join up to 275 being positioned in and around Iraq to provide security and support for the U.S. Embassy in Baghdad and other American interests.

But he was adamant that U.S. troops would not be returning to combat.

Despite the deteriorating conditions, Obama has held off approving airstrikes sought by the Iraqi government. The president said he could still approve "targeted and precise" strikes if the situation on the ground required it, noting that the U.S. had stepped up intelligence gathering in Iraq to help identify potential targets.

U.S. flights over Iraq 24 hours a day

U.S. officials say manned and unmanned U.S. aircraft are now flying over Iraq 24 hours a day on intelligence collection missions.

Not all Shias welcomed the announcement that more Americans were heading to Iraq.

A Shia cleric, Nassir al-Saedi, warned that the 300 advisers would be attacked. Al-Saedi is loyal to anti-U.S. cleric Muqtada al-Sadr, whose Mahdi Army militia fought the Americans in at least two rounds of street warfare during their eight-year presence in Iraq.

"Our message to the occupier: ... We will be ready for you if you are back," he told a Friday sermon attended by al-Sadr supporters in Baghdad's Sadr City district.

Mohammed al-Khalidi, a Sunni lawmaker who favours replacing al-Maliki's government with a more inclusive one involving Shias, Sunnis and Kurds, said he thought "Obama's statement was balanced and reasonable."

"But," he added, "U.S. officials should be aware that the situation in Iraq needs an immediate remedy because Iraq is heading to the unknown."