Reports from Lebanon say hundreds of Iranian troops have arrived in Syria in the last 10 days and will soon join government forces and their Lebanese Hezbollah allies in a major ground offensive backed by Russian air strikes.
Reuters news agency reported on Thursday the arrival of the Iranians, quoting two Lebanese sources:
"The [Russian] air strikes will, in the near future, be accompanied by ground advances by the Syrian army and its allies," said one of the sources familiar with political and military developments in the conflict.
"It is possible that the coming land operations will be focused in the Idlib and Hama countryside."
Al Jazeera could not independently verify the report.
The two Reuters sources said the operation will be aimed at recapturing territory lost by President Bashar al-Assad's government to rebels.
If true, the development points to an emerging military alliance between Russia and Assad's other main allies - Iran and Hezbollah - focused on recapturing areas of northwestern Syria that were seized by opposition fighters in rapid advances earlier this year.
"The vanguard of Iranian ground forces began arriving in Syria: soldiers and officers specifically to participate in this battle," the second source said.
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"They are not advisers ... we mean hundreds with equipment and weapons. They will be followed by more."
Iraqis will also take part in the operation, the source said.
Thus far, direct Iranian military support for Assad has come mostly in the form of military advisers.
Iran has also mobilised Shia units, including Iraqis and some Afghans, to fight alongside Syrian government forces.
Hezbollah, the Iran-backed Lebanese Shia group, has been fighting alongside the Syrian army since early in the conflict.
The White House said it could not confirm reports that Iranian troops have launched a ground offensive in Syria, but said any such development would be an "apt and powerful illustration" that Russia's military actions have worsened the conflict.
Josh Earnest, White House spokesman, that Russian involvement has not caused a "broad re-evaluation" of US strategy in Syria.
The Russian air force began air strikes in Syria on Wednesday, targeting areas near the cities of Homs and Hama in the west of the country, where Assad's forces are fighting an array of rebel groups.
The stated target of the Russian air strikes, the Islamic State of Iraq and the Levant (ISIL) group, is based mostly in the north and east.
An alliance of opposition groups, including the Nusra Front and Ahrar al-Sham, made rapid gains in Idlib province earlier this year, completely expelling the government from the area bordering Turkey.