Hundreds of Iran-backed Iraqi fighters have crossed into Syria to support President Bashar al-Assad's forces following recent rebel advances, according to multiple security sources in Baghdad, marking a significant escalation in regional involvement in Syria's conflict.

At least 300 fighters from Iraq's Badr Organisation and Al-Nujaba Movement crossed into Syria late December 3 using dirt roads to avoid official border crossings, two Iraqi security sources speaking with IntelliNews.

The fighters moved in small groups to avoid airstrikes, according to a senior Syrian military source.

"These are new reinforcements being sent to assist our comrades on the northern front lines," the Syrian military source said, speaking on condition of anonymity. The Iraqi groups claimed they were deploying to defend a Shi'ite shrine, though their movement comes as Assad's forces face mounting pressure.

The development highlights how regional alliances are shifting as Assad's government struggles to maintain control following the rebels' surprise capture of Aleppo.

Iran-aligned regional militias, supported by Russian air power, had previously played a crucial role in helping government forces subdue the rebellion that began in 2011.

However, this coalition faces fresh challenges as Russia focuses on Ukraine and Lebanon's Hezbollah grapples with its conflict with Israel.

Hadi al-Bahra, leader of the main Syrian opposition abroad, said rebels captured Aleppo rapidly because Hezbollah and other Iran-backed groups were preoccupied with the Israel conflict.

Three sources familiar with Hezbollah's thinking said the group has not been asked to intervene and remains reluctant to deploy forces following its intensive engagement with Israel.

The group withdrew senior officers responsible for Aleppo to assist in ground operations against Israel prior to last week's ceasefire, one source said.

The Israeli military has warned it will not allow Iran to exploit the Syrian conflict to transfer weapons to Hezbollah.

Two sources, one Lebanese and one Syrian, confirmed Hezbollah withdrew its forces from Syria in mid-October as fighting with Israel intensified.