This image shows homes destroyed by government airstrikes and shelling, in the Barzeh district of Damascus, Syria on Saturday, June 1, 2013. (HOEP/AP)

Thousands of Lebanese Hezbollah militants were massed around the northern Syrian city of Aleppo on Sunday, according to rebels and a senior commander in the Lebanese Shiite movement, broadening Hezbollah’s backing of President Bashar al-Assad’s forces and stoking fears of an imminent assault on the city.

The commander, who spoke on the condition of anonymity because he is not authorized to talk to the media, said there were about 2,000 Hezbollah fighters in Aleppo province, largely stationed in Shiite towns north of the city. The rebel Free Syrian Army said Hezbollah forces had gathered in a suburb of the city Sunday and appeared to be preparing for an attack.

Rebels have secured swaths of Aleppo — Syria’s commercial capital and most populous city — since fighting engulfed it last summer, but the two sides have been locked in a grinding stalemate for months. An assault on the city could stretch rebel forces, which have sent reinforcements from Aleppo to fight against Hezbollah and Syrian troops in the battle for the town of Qusair, near the Lebanese border.

The claims of a Hezbollah presence in Syria’s north follow a pledge by its leader, Hasan Nasrallah, to back Assad to victory and indicate that the movement could be used as a guerrilla force wherever required. A long-standing ally of Syria and Iran, its decision to knuckle into the fight raises the specter of a regional conflagration spilling over Syria’s borders, pitting Sunni against Shiite. Underscoring that point, Syrian rebels and Hezbollah fighters engaged Sunday in their first serious clashes on Lebanese soil.

“The Aleppo battle has started on a very small scale; we’ve only just entered the game,” said the Hezbollah commander in an interview in Beirut on Saturday while on leave from fighting in Qusair, where he oversees five units. “We are going to go after strongholds where they think they are safe. They are going to fall like dominoes.”

He said the militants were largely concentrated around the Shiite towns of Zahra and Nubol, which have been under siege from largely Sunni rebel forces. A spokesman for Hezbollah said he could not confirm or deny their presence.

Louay al-Mokdad, political and media coordinator for the Free Syrian Army, said Hezbollah militants had gathered at a military academy in Aleppo’s western district of Hamdaniyah on Sunday. He put the number of the Shiite movement’s soldiers in the area at 4,000, quoting rebel intelligence.

“We think they are going to engage inside Aleppo and the province,” he said.

In what appeared to preparation for that, pro-government forces began a push to secure supply lines to the city on Sunday, activists said. Aleppo-based activist Kareem Abeed said pro-government forces had advanced from the military academy in Hamdaniyah, with rebels repelling an attack in the Rashideen neighborhood.

The infiltration of Hezbollah fighters into Syria — along with the supply of weapons from Russia and Iran — has helped turn the tide in favor of Assad’s government, Sen. John McCain (R-Ariz.) said Sunday.

“We are seeing, unfortunately, a battlefield situation where Bashar al-Assad now has the upper hand, and it’s tragic,” McCain, who slipped into Syria last week to meet with rebel fighters, said on CBS’s “Face the Nation.”

McCain, who has repeatedly called for military action in Syria and who has been among the harshest critics of the Obama administration on the issue, recalled claims from U.S. officials dating back more than year ago that Assad’s fall was inevitable.

“I think we can’t make that statement today,” he said. “Hezbollah [has] now invaded. The Iranians are there. Russia is pouring weapons in. And anybody that believes that Bashar Assad is going to go to a conference in Geneva when he is prevailing on the battlefield — it’s just ludicrous to assume that.”

McCain was referring to an international conference planned for this month or possibly July to bring the warring sides together. The Syrian opposition has said it will not attend while Hezbollah’s siege of Qusair continues.

The siege showed no sign of abating Sunday, as Syrian Foreign Minister Walid al-Moualem rejected a request from the United Nations to allow the International Committee of the Red Cross to enter the town immediately and tend to an estimated 1,500 wounded trapped inside.

The Hezbollah commander boasted about gains in Qusair, saying that when he left the battlefield for leave a week ago, the movement controlled 70 percent of the city at the cost of 72 of its men. He said 3,000 Hezbollah fighters are in the town, among “no more than 10,000” in the whole of Syria.

However, Sami al-Rifaie, an activist based in Qusair, said rebels have made gains since reinforcements arrived, with Hezbollah and army control reduced to 20 percent of the city.

Liwa al-Tawhid Brigade, one of the largest opposition groups in the area, has sent men from Aleppo to back embattled rebels in Qusair.

In a sign that Hezbollah may be under more strain than expected, the commander said that seven-days-on, seven-days-off military rotations have been changed to 20 days on before a week-long leave.

Justifying Hezbollah’s involvement in the Syrian conflict, Nasrallah has painted the largely Sunni opposition to Assad as extremist Muslims backed by the United States and Israel, Hezbollah’s long-standing enemy. He has warned that they will eventually invade Lebanon if they are not put down across the border.

But even after announcing all-out backing for Assad, Hezbollah fighters had been largely confined to Qusair, which is just a few miles from Lebanon, and in Damascus suburbs around the Shiite shrine of Sayyida Zeinab, which it has pledged to protect.

In a video posted online Saturday, a battalion of the Liwa al-Tawhid Brigade declared it was leaving for Zahra and Nubol to fight the “party of the devil,” a term often used by rebels to refer to Hezbollah, which translates as Party of God.

If Hezbollah is present in Aleppo, it is plausible that it could be utilized anywhere in the country, said Emile Hokayem, a Middle East-based analyst at the International Institute for Strategic Studies.

“A deployment so deep into Syria and in such a crucial place would be a clear indication that Hezbollah’s role in Syria was never limited to defensive aims but is geared toward helping Assad score major victories,” he said.

Hezbollah’s entanglement in Syria has sparked a backlash within fragile Lebanon, with Syrian rebels firing rockets on Shiite areas of the country with increasing frequency in recent weeks.

On Sunday, according to Lebanese security individuals quoted by the Reuters news agency, one member of Hezbollah and at least 12 rebels were killed in clashes in Ain el-Jaouze, a finger of Lebanese territory that juts into Syria, near Lebanon’s Bekaa Valley town of Baalbek. The men may have been ambushed by Hezbollah as they tried to fire rockets at Shiite areas of the Bekaa Valley, they said.

“The presence of Hezbollah units around Aleppo will only deepen the divide in Lebanon and confirm, in the eyes of its rivals, Hezbollah’s complete alignment with Assad,” Hokayem said.

Ahmed Ramadan and Suzan Haidamous contributed to this report.