Iraq's highest Shia authority, Ayatollah Ali al-Sistani, has called for the quick formation of a new and "effective" government in a move that piles more pressure on the country's embattled leader, Nouri al-Maliki.

Although he avoided directly criticising the Iraqi prime minister, Sistani's call is far short of the resounding support Maliki needs to overcome rising unease at home over his leadership as well as rapidly shrinking international support.

Sistani also renewed a demand he made last week for his followers, who comprise the majority of Iraq's dominant Shia sect, to fight the jihadist group Isis and its insurgency that continues to ravage north and central Iraq.

"They must be fought and expelled from Iraq, [or] everyone will regret it tomorrow, when regret has no meaning," Sistani's spokesman announced during Friday prayers in Najaf.

After a disastrous response to the Isis push nine days ago, Iraqi troops are now holding ground in three battlefields, Baquba 60 miles north of Iraq, the vital Baiji oil refinery near Tikrit and the city of Tal Afar northwest of Mosul, where soldiers have re-entered part of the city taken by Isis on Tuesday.

However, the military remains unable to shift Isis from its strongholds or reverse the gains the group made during a stunning sweep through Mosul and Tikrit that continues to pose a grave threat to Iraq's borders.

American military advisers are understood to be en route to the country, as are marines sent to defend the US Embassy, along with Australian special forces deployed to perform the same role at their embassy. However, Barack Obama insisted that none would play a ground forces role. The US president also appears to be placing the condition on Iraqi demands to send air support that Maliki step aside first.

After elections eight weeks ago, Maliki has been trying to assemble a coalition government, which he would lead as a third-term prime minister. Though his bloc emerged with the largest number of seats in the 328-seat parliament, it is difficult to see from where he could muster the numbers he needs.

US officials visiting Baghdad this week have stressed the need to form a government urgently to avoid yet another power vacuum taking hold at a time when Iraq can least afford it.

In another sign that the state now has several powerful rivals to its authority, the Jeish al-Mehdi, the most feared arm of the Shia insurgency during the almost nine years the US occupied Iraq, will take to the streets of Baghdad, Basra, Najaf and Karbala on Saturday in a display of military might.

Shia irregulars are playing prominent roles in the fight against Isis. Though loosely aligned to the military, they have their own leadership structures and commanders. Many see the fight through the lens of their sectarian identity, with nationalistic considerations running second.

Isis has vowed to oust Iraq's Shia powerbase and to collapse the state's borders at it attempts to reimpose a hardline 7th-century Islamic caliphate on the lands of both Syria and Iraq. It has drawn support from former Ba'athists who were disempowered by the fall of Saddam Hussein in 2003 and have been trying to reorganise ever since.