Iraq's top Shiite cleric has urged all of its people to unite and expel Sunni Muslim insurgents, as Prime Minister Nuri al-Maliki came under growing pressure at home and abroad.

The call came Friday after US President Barack Obama stopped short of acceding to Maliki's appeal for air strikes against militants, including the Islamic State of Iraq and the Levant (ISIL), prompting neighbouring Shiite Iran to charge that Washington lacked the "will" to fight terror.

A swift militant offensive, led by the jihadist ISIL, has overrun swathes of northern and central Iraq, risking the United States' already damaged legacy there, displacing hundreds of thousands and threatening the country's very existence.

Grand Ayatollah Ali al-Sistani, a revered cleric among the Shiite majority, called on Iraqis to band together against the insurgents before it was too late.

If ISIL is not "fought and expelled from Iraq, everyone will regret it tomorrow, when regret has no meaning", his spokesman announced on his behalf.

The reclusive Sistani, who heads a council of senior clerics, said Iraq's next government must be "effective" and avoid "past mistakes", an apparent rebuke to Maliki, premier since 2006.

His remarks came after several senior American figures pushed Maliki, seeking to retain the premiership after winning a plurality in April 30 elections, to work with Iraq's Sunni Arab and Kurdish minorities.

US Vice President Joe Biden, Defence Secretary Chuck Hagel, Chairman of the Joint Chiefs General Martin Dempsey and David Petraeus, the former top US commander in Iraq, have all either called for Maliki to be more inclusive or outright criticised him.

- 'Definite uptick' -

With Secretary of State John Kerry heading to Europe and the Middle East for talks on Iraq, Obama said Thursday that Maliki's actions could dictate the country's fate, amid a growing feeling in Washington that the Iraqi leader would do best by moving on.

Former US ambassador to Baghdad James Jeffrey said there has been a "definite uptick" in Washington's criticism of Maliki.

Obama, who based his political career on ending the costly eight-year US intervention in Iraq, has insisted Washington is not slipping back into the morass.

But he has offered as many as 300 military advisers and left open the possibility of "targeted and precise military action if and when we determine that the situation on the ground requires it".

Washington already has an aircraft carrier in the Gulf and is flying manned and unmanned surveillance flights over Iraq.

Senior US officials said privately special forces being sent to advise Iraqi forces could call in air strikes if necessary.

The latest offer was the most concrete action announced by Washington since the crisis erupted on June 9.

But it fell short of Iraq's request for air strikes and drew derision from Tehran, which has offered its cooperation despite decades of enmity but also, according to US, sent a "small number" of operatives into its neighbour.

- 'Lacks serious will' -

"Obama's comments show the White House lacks serious will in fighting terrorism in Iraq and the region," Deputy Foreign Minister Hossein Amir Abdollahian said.

Obama llater told CNN that "no amount of American firepower" could keep Iraq together unless its political leaders reject sectarianism and work to unite the country.

"We gave Iraq the chance to have an inclusive democracy. To work across sectarian lines to provide a better future for their children. And unfortunately what we've seen is a breakdown of trust," he said.

Russian President Vladimir Putin, meanwhile, offered Moscow's "complete support" in a telephone call with Maliki.

UN chief Ban Ki-moon warned that military strikes against the jihadists could prove counterproductive without any movement toward inclusive government.

Earlier this week he said the conflict threatened to spill over Iraq's borders, interlocking with the civil war in neighbouring Syria, where ISIL is also fighting.

Thirty-four members of the security forces were killed in a town on the border Friday, while 30 pro-government Shiite militiamen died in a firefight with insurgents northeast of the capital in Diyala province.

Elsewhere, the battle for the strategic northern town of Tal Afar entered its sixth day. Witnesses said security forces clashed with militants, who still hold significant ground.

Shiite-majority Tal Afar is located along a strategic corridor to Syria, and the largest town not to fall to militants in the northern province of Nineveh, most of which has been overrun.

An army lieutenant general, asking not to be identified, told AFP that Iraq needs US help "to stop terrorism and eliminate it... especially through air strikes against specific targets".

Meanwhile, UN aid agencies said they were rushing supplies to Iraq to help more than one million people displaced by the current fighting and unrest earlier this year.

Around half of those fled after the northern city of Mosul fell to ISIL last week, and tens of thousands have hit the roads in the eastern province of Diyala, near Baghdad, and Salaheddin, north of the capital.