The assault seemed to focus on an area straddling northern Hama Province and southern Idlib Province, where insurgent command of high ground threatens the coast. The initial ground attacks took place around three villages that insurgents consider the first line of defense of the strategic Jebel al-Zawiyah area.

The bombardment appeared to reach new levels of intensity in some places. One video showed white smoke rising far 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.

A number of times in Wednesday’s fighting, insurgents 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 UnitedStates. Rebel groups, including two that have received American aid, Division 13 and Suqour al-Ghab, posted videos that showed the guided missiles sailing toward approaching tanks and destroying them.

The main thrust of the offensive was aimed at areas held by insurgent groups that oppose both Mr. Assad and the Islamic State, including the Nusra Front, Al Qaeda’s Syrian affiliate. But there were airstrikes elsewhere in Syria, according to SANA, the state news agency, which said that Syrian and Russian warplanes had worked together to attack targets in Al Bab, a city in eastern Aleppo Province long held by the Islamic State.

While Russian officials said the missiles launched from the Caspian Sea had targeted the Islamic State, also called ISIS or ISIL, Western officials said the great majority of