As the head of the Iranian Revolutionary Guards' Quds Force, Soleimani oversaw Tehran's interventions in regional power struggles from Lebanon and Iraq to Syria and Yemen.

Washington had hoped his killing in a Baghdad drone strike Friday would deal a blow to Iran and its network of proxies -- but the plan appears to have backfired by uniting pro-Iran factions under an "axis of resistance".

"The strike unified the resistance forces and made combatting the United States a priority," said Qassem Qassir, a Lebanese expert in Islamic movements.

"The assassination was a strategic mistake, and the response will be across the region -- not just limited to Iraq," said Qassir.

Indeed, pro-Iran factions in Iraq have seized on the strike to secure a political and popular revival.

Kataeb Hezbollah, a vehemently anti-American armed faction in Iraq, said the strike was "the beginning of the end of the US presence in the region".

Iraqi populist cleric Moqtada Sadr swiftly reactivated his Mahdi Army, the notorious militia that fought US troops after the American-led invasion of 2003.

"The Iraqi factions of the resistance must hold an immediate meeting to form the International Resistance Regiments," he tweeted, telling his fighters to "be ready".

- 'Turn the skies to hell' -

Qais al-Khazali, a paramilitary leader and bitter rival of Sadr's, echoed his calls for fighting units to mobilise following the strike on Soleimani.

Khazali also threatened US troops who have been stationed across Iraq since 2014 as part of the global coalition battling the Islamic State group.

On Sunday, Iraq's parliament voted in favour of ousting US troops although the decision rests with the government.

"If you don't leave, or if you procrastinate in leaving, you will find a strong Iraqi response that will shake the ground beneath your feat and turn the skies above you into hell," Khazali warned.

Even Grand Ayatollah Ali Sistani, Iraq's highest Shiite authority, broke with standard protocol to mourn Soleimani.

In a first, Sistani sent a letter to Iran's Supreme Leader Ali Khamenei to offer his condolences.

Further afield in Lebanon, Iran-backed Hezbollah said the strike represented a threat to "all the movements, leaders and countries of the axis of resistance".

The killing of "Qasem Soleimani is not an Iranian issue. It concerns the axis of resistance -- it concerns the Muslim world," said the movement's influential head Hassan Nasrallah.

Iran-backed Houthi rebels in Yemen, meanwhile, called for "direct and swift reprisals" to the strike.

Palestinian movement Hamas slammed it as an "American rampage," and its head Ismail Haniya travelled to Tehran for Soleimani's funeral.

And the leftist Popular Front for the Liberation of Palestine also urged "a coordinated, comprehensive and continuous response from resistance forces".

- 'Closing of rank' -

"There could be a closing of rank and a reinforcement of the confessionalism," said Karim Bitar of the Paris-based Institute for International and Strategic Affairs.

He said Iran's allies in the region would set economic or political goals aside to prioritise the "emergency security situation" triggered by Soleimani's death.

"Soon enough, this decision by (US President) Donald Trump will be seen as counter-productive," Bitar predicted.

Trump has threatened Iran with "major retaliation" if it responds to the strike, openly warning in a tweet on Sunday that US action may even be "disproportionate".

He had already threatened to bomb 52 unspecified targets in Iran if Tehran attacks US interests in the region.

"Tehran has the sword of Damocles hanging over its head," Bitar told AFP.

"But the threat of foreign intervention will reunite Iranians of all social classes, both opponents and supporters of the regime," he said.

Indeed, unprecedented crowds have turned out in Iran to mourn Soleimani and the four other Revolutionary Guards killed in the US strike.

Ultimately, the assassination could end up bolstering the Iranian government, which will benefit from a phenomenon of "rallying around the flag," Bitar said.