A Free Syrian Army fighter rides on a motorbike with his children past the rubble of damaged buildings Oct. 18 in the old city of Aleppo. (Hosam Katan/Reuters)

In a striking sign of Iran’s growing regional influence, a major assault on Syria’s most populous city is being coordinated by an Iranian military commander using Shiite forces from three countries to back President Bashar al-Assad’s beleaguered troops, militia officials said.

Maj. Gen. Qasem Soleimani, the leader of Iran’s elite Quds Force, has ordered thousands of Iraqi Shiite militia allies into Syria for the operation to recapture Aleppo, according to officials from three of the militias. The militiamen are to join Iranian troops and forces from Hezbollah, the Iranian-backed Lebanese militia, the officials said.

Soleimani has been a frequent sight on the battlefields in neighboring Iraq, where he has been advising Iraqi forces fighting Islamic State militants. But the war there has stagnated, and the shift of the commander along with Iraqi militiamen and Quds Force members to Syria appears to signal a change in Iranian priorities.

The battle for Sunni-majority Aleppo, announced Friday by Syria, is important because of the city’s size and prominence as an economic hub before the outbreak of Syria’s civil war. Its eastern side has been held by rebels since 2012.

Russia is also involved in the fight for the city, launching airstrikes in the vicinity to support Assad’s forces.

A girl crosses a street as damaged buses are positioned atop a building as barricades to provide protection from snipers of the forces of Syria's President Bashar al-Assad. (Hosam Katan/Reuters)

Pro-government forces have claimed victory in a string of villages near Aleppo in recent days.

Phillip Smyth, a researcher on Shiite groups at the University of Maryland, said Iran is clearly increasing its participation on the front lines of the Syrian conflict.

“It’s not only one of the largest open deployments [by Iran and its proxies during the war] but it has also involved one of the largest Iraq-focused Shia militia recruitment efforts for Syria in years,” Smyth said.

Shiite militias frame the Aleppo fight as part of a single regional struggle pitting Shiites against Sunni extremists from the Islamic State group, who have overrun large swaths of Iraq and Syria.

“It makes no difference whether we’re in Iraq or Syria, we consider it the same front line because we are fighting the same enemy,” said Bashar al-Saidi, a spokesman for Harakat Hezbollah al-Nujaba, an Iraqi Shiite militia that says it has fighters around Aleppo. “We are all the followers of Khamenei and will go and fight to defend the holy sites and Shiites everywhere,” he said, referring to Iran’s supreme leader, Ayatollah Ali Khamenei.

Saidi said the group has been sending reinforcements to Syria for several months but declined to give numbers.

Iraq’s Shiite militias have had a presence in Syria since 2013, but the majority of fighters were pulled back to Iraq last summer, as the Islamic State made rapid advances in the country and threatened the capital, Baghdad.

Civilians walk past sanitation cylinders placed as barricades to provide protection from Syrian snipers in the old city of Aleppo. (Hosam Katan/Reuters)

However, with Syrian troops struggling and forces from Hezbollah, Iran’s proxy in Lebanon, also suffering high rates of attrition, Iranian-backed Iraqi militias have been asked to provide reinforcements.

“The road to liberate Mosul is through Aleppo,” Saidi said Soleimani told him, referring to the Iraqi city that was seized by Islamic State militants in June 2014 and that has become the group’s de facto capital in Iraq.

Hezbollah and the Quds Force, which is part of Iran’s Revolutionary Guard Corps, have also sent reinforcements, he said. Two other Iraqi militia officials confirmed the deployments. Last week, a U.S. defense official said hundreds of Iranian troops were near Aleppo in preparation for an offensive — the first confirmation of Iranian ground troops, though the country has long had military advisers Syria.

“It’s not a secret,” Saidi said. “We are all fighting against the same enemy,” .

His militia released a photo last week of Soleimani, the Quds Force commander, with its fighters near Aleppo.

In August, U.S. officials raised concerns with Russia about news that Soleimani, who was subject to travel restrictions under U.N. sanctions, had gone to Moscow in July to meet with President Vladimir Putin. Just short of two months later, both countries were escalating their military involvement in Syria in support of Assad.

Kitaeb Hezbollah, an Iranian-backed Shiite militia designated a terrorist organization by the United States, has sent 1,000 troops to Aleppo , said a senior official with the militia. He spoke on the condition of anonymity, citing orders that the deployment not be made public yet.

He said the men were part of the group’s elite forces, which have experience from fighting the United States in Iraq. They have done previous rotations in Syria, he said.

“They were sent based on a demand from Soleimani,” he said. “He specifically requested them for the launch of the operation of Aleppo, which is going to be led by Kitaeb directly under the supervision of Soleimani.”

The militia official said the Syrian army would have a “minor role.”

The movement of fighters away from Iraq comes as the battlefield there stagnates. Iranian-backed proxy forces have largely been kept out of efforts to retake the western Iraqi city of Ramadi, seized by the Islamic State in May. The U.S. and Iraqi governments have raised concerns about the Shiite forces leading operations in a Sunni city.

While officials in Moscow have attempted to portray their military escalation in Syria as directed against Islamic State militants, those insurgents have gained in the Aleppo area since Russia’s airstrikes began. Islamic State militants have pushed toward the city from the north, as pro-government forces move from the south.

The rebel-held areas in and around the city are some of the few in Syria that remain in the hands of more moderate groups, but they contain a mosaic of opposition factions.

The Kitaeb Hezbollah official said that despite advances in recent days by pro-government forces, an Aleppo offensive has yet to begin in earnest.

The 1,000 Kitaeb Hezbollah fighters “are our special forces and well trained from fighting against the Americans before, and in Syria and Iraq,” he said.

“The operation is an extension for our operations in Iraq because it’s the same enemy, and when we hit them there it means that it will get results in Iraqi lands,” the Kitaeb official said.

Hasan Abdul-Hadi, a spokesman for Kitaeb Sayyid al-Shuhada, another Iraqi militia, said his group has 500 fighters in Syria — in Aleppo and Daraa, in the far southwest.

“Soleimani is the one who coordinates the resistance factions,” he said, “and the Iranians support us with weapons.”

Salim reported from Baghdad. Suzan Haidamous in Beirut contributed to this report.

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