The two blocs won first and second place in the war-scarred country's May 12 parliamentary election.

The move by Sadr, who is staunchly opposed to Iranian involvement in the country, was unexpected by much of the political class as he had suggested unwillingness to work with Ameri and his bloc of pro-Iranian former fighters.

But at a joint press conference with Ameri in the Shiite holy city of Najaf, Sadr hailed the formation of "a true alliance to accelerate the formation of a national government away from any dogmatism".

Confusion has gripped Iraq since Sadr's electoral alliance with communists won the vote. Last week Iraq's parliament ordered a manual vote recount and sacked the commission which oversaw the polls amid mounting claims of electoral fraud.

"This is a call to all those who care about national interests... we will set up committees to discuss with all ways to accelerate the drafting of a government program," said Ameri, whose group played a key role last year in the defeat of the Islamic State group.

The May election saw a record number of abstentions as many Iraqis snubbed the corruption-tainted elite that had dominated the country since the 2003 US-led invasion that toppled Saddam Hussein.

Many of Iraq's longtime political figures -- seemingly irremovable since the dictator's fall -- were pushed out of their seats by new faces.

Sadr, a former militia leader who led two uprisings against the US-led invasion of Iraq, has called for his country to be more independent from both Iran and the US.

By forming a coalition with Ameri, Sadr moves a step closer to getting enough seats to generate a majority in the 329 seat parliament.