BEIRUT, Lebanon — Russia and Syria unleashed a coordinated assault by land, air and sea on Wednesday, seeking to reverse recent gains by rebel groups that were beginning to encroach on President Bashar al-Assad’s last bastion of power.

Moscow said it had fired 26 cruise missiles on Syrian targets from naval vessels in the Caspian Sea, 900 miles away, though it was not immediately clear whether they hit targets in the area of the ground offensive.

The ground assault, and airstrikes, seemed to focus on an area of northern Hama Province and southern Idlib Province, around three villages that insurgents consider the first line of defense of the strategic Jebel al-Zawiyah area.

The bombardment appeared to be severe in some places. One video showed white smoke rising above a village’s minarets, while another appeared to show at least a dozen explosions — the person filming described the weapons as rockets — in less than five minutes.

At certain points in Wednesday’s fighting, rebels fired advanced TOW antitank missiles, supplied covertly by the C.I.A., at Syria’s Russian-made tanks, leaving the impression of a proxy war between Russia and the United States. Videos posted by rebel groups, including the American-backed Division 17 and Suqour al-Ghab, showed the guided missiles sailing toward approaching tanks and destroying them.  (read more)