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, two Lebanese sources told Reuters.
"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," the source added.
The two sources said the operation would be aimed at recapturing territory lost by President Bashar al-Assad's government to rebels.
It points to an emerging military alliance between Russia and Mr. Assad's other main allies - Iran and Hezbollah - focused on recapturing areas of north-western Syria that were seized by insurgents 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. They are not advisors ... we mean hundreds with equipment and weapons. They will be followed by more," the second source said. Iraqis would also take part in the operation, the source said.
Thus far, direct Iranian military support for Mr. Assad has come mostly in the form of military advisors. Iran has also mobilised Shia militia fighters, including Iraqis and some Afghans, to fight alongside Syrian government forces.
Lebanon's Hezbollah, which is backed by Iran, has been fighting alongside the Syrian army since early in the conflict.
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 Mr. Assad's forces are fighting an array of insurgent groups, though not Islamic State, which is based mostly in the north and east.
An alliance of insurgent groups including the al Qaeda-linked Nusra Front and powerful Ahrar al-Sham made rapid gains in Idlib province earlier this year, completely expelling the government from the area bordering Turkey.
