"Our meeting was a very positive one, we met to end the suffering of this nation and of the people. Our new alliance is a nationalist one," Sadr said.
Sadr and Amiri are strange bedfellows.
The cleric, who once led violent campaigns against the U.S. occupation that ended in 2011, has emerged as a nationalist opponent of powerful Shi'ite parties allied with neighbouring Iran and as a champion of the poor.
He backed in the election the Saeroon list composed of his followers, the Communist Party, and other secular candidates.
Amiri, a fluent Farsi speaker, is Iran's closest ally in Iraq, having spent two years in exile there during the era of Saddam Hussein.
The Fatih alliance he led in the election was composed of political groups tied to Iran-backed Shi'ite militias who helped government forces dislodge Islamic State militants from the third of Iraq they seized four years ago.
"Fatih and Saeroon announce forming the nucleus of the largest bloc and call on all winning blocs to participate in this alliance under a government programme agreed upon by all that is suitable to face the challenges, crises, and problems facing Iraq," a Fatih spokesman said in a statement.
The alliance, which together has 101 seats, 64 short of the majority needed to form a government, came hours after Prime Minister Haider al-Abadi, who's own bloc came third, urged politicians to keep negotiating over government formation despite an impending nationwide manual recount of votes.
Parliament mandated the recount after Abadi said a government report showed there were serious violations.
A few days later a storage site housing half of Baghdad's ballot boxes caught fire, raising tensions and prompting some to call for the election to be repeated.
Abadi said on Tuesday he opposed a repeat, echoing the stances of Sadr and Amiri, and warned that anyone who tried to sabotage the political process would be punished.
He may yet secure a second term as a compromise candidate if he joins his blocs with Sadr and Amiri and manages to win their backing.
"The matter is exclusively in the hands of the judiciary, not politicians. The government and parliament don't have the power to cancel the election," Abadi said of a repeat.
The government report had recommended a recount of 5 percent of votes but the outgoing parliament, in which over half of lawmakers including the speaker lost their seats, instead voted for a nationwide one.
Amiri said on Tuesday he supported only a partial recount.
Abadi called the fire a deliberate act and said the attorney general would bring charges against those who are trying to undermine the political process.
An Iraqi court ordered the arrest of four people accused of setting fire to the storage site. Three of them were policemen and one an employee of the elections commission.
THREAT OF VIOLENCE
Abadi said a preliminary report had provided evidence of gasoline at multiple areas inside the storage site. It also showed that security cameras had been disabled and no locks had been broken, implying it was carried out by someone with access to the storage site.
Iraqi authorities said the ballot boxes had been rescued but the fire has fuelled fears of violence.
Sadr has warned that certain parties are trying to drag Iraq into a civil war, adding that he would not participate in one. urn:newsml:reuters.com:*:nL8N1TD1LF
Abadi thanked Sadr for a disarmament initiative he floated after a weapons cache at his Baghdad stronghold of Sadr City exploded, killing 18 people, and said he hoped the cleric would stick to it.
"I welcome Sayed Moqtada's announcement that his followers commit to not having weapons outside the framework of the state. We consider this good," he said, adding that those responsible for the explosion would be brought to justice.
"What happened in Sadr City is very regrettable, it is a crime. Those responsible will receive their just punishment."
Sadr, who ordered his own separate investigation into the incident, said on Tuesday he had identified the culprit who was now on the run and that he would be brought to justice.
(Reporting by Ahmed Aboulenein; additional reporting by Huda Majeed Editing by Matthew Mpoke Bigg) ((email@example.com; +964 790 191 7021; Reuters Messaging: firstname.lastname@example.org))