"There are terrorist groups that have completed their preparations for a breakthrough into Baghdad," the cleric said in a statement.

"We are ready to defend the city, we are ready to supply forces and coordinate with the authorities to face any scenario," said Sadr, who announced the creation of the Saraya al-Salam (Peace Brigades) group in the aftermath of the extremists’ offensive that began in June.

The force includes former fighters from his now dissolved Mahdi Army that fought US occupation 10 years ago.

US jets were back in Iraqi skies Friday after President Barack Obama announced he had ordered food drops to civilians stranded in the north after being displaced by extremists’ attacks.

He also said he authorized air strikes against ISIL positions to ensure the safety of US assets in Kurdistan, after the extremist militants moved within striking distance of the autonomous Kurdish region.

The Iraqi army chief of staff, Lieutenant General Babaker Zebari, told AFP he thought US jets would extend their campaign against other towns held by ISIL but he did not say which ones.

ISIL has controlled parts of Syria for some time and while it had bases in Iraq, it launched a major offensive on June 9 which saw it conquer much of Iraq's north and allow for the proclamation of a "caliphate" straddling both countries.

Extremists stopped their offensive a few dozens kilometers from Baghdad and experts have generally said they may be waiting to gather more power for an all-out attack on capital.

SHI/SHI