Iranian troops have arrived in Syria to join Syrian President Bashar al-Assad's forces to launch a ground attack against the Islamic State militant group in the war-torn country.

"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," a Lebanese source told Reuters. "The (Russian) air strikes will in the near future be accompanied by ground advances by the Syrian army and its allies. It is possible that the coming land operations will be focused in the Idlib and Hama countryside."

Assad's staunch ally Russia launched its first air strikes in Syria on Wednesday. Russians are targeting ISIS's locations in Raqqa and Deir Ezzour provinces and are likely to provide air cover to Iranian ground forces. Watch the footage of Russian airstrikes in Syria below.

"They want to clean the country of non-ISIL rebels, and then the U.S. will work with them as ISIL will be the only enemy," the source said, according to the National Post.

The U.S., however, declined to confirm reports about Iranian ground forces' involvement in Syria. But it slammed Russia for its military intervention in Syria.

"If true, and the reports are coming from a variety of sources, it would be a rather apt and even powerful illustration of how Russia military intervention inside of Syria, focused on an indiscriminate bombing of Syrian opposition targets has worsened the sectarian conflict there," White House Press Secretary Josh Earnest told reporters, according to Euronews.