The Syrian Army has launched a long-planned offensive in northern Homs Province.
Russian warplanes are reported to be taking part in the Syrian Army assault on rebel-held towns north of the city of Homs.
The Syrian Observatory for Human Rights -- which monitors the war -- said Russian air strikes on October 15 hit targets around the town of Talbiseh and Teir Malla, about five kilometers north of the mainly government-held city.
The U.K.-based group said Syrian soldiers and fighters from Lebanon's Hizballah were involved in the military operation.
Russia has stepped up its air strikes in recent days, announcing on October 13 that it had carried out 88 missions in the previous 24 hours, one of the heaviest days of bombardment of its campaign so far.
While Moscow says it is bombing Islamic State targets, Washington and its allies accuse it of mainly targeting moderate Western-backed rebels to prop up the regime of President Bashar al-Assad, a longtime Russian ally.
Moscow's intervention means Russian and U.S. jets are flying combat missions over the same country, raising fears of accidental confrontation.
The militaries of the United States and Russia have been in talks aimed at avoiding accidental collisions over Syria.
On October 14, the White House dismissed a proposal by Russian President Vladimir Putin to send Prime Minister Dmitry Medvedev to the United States to discuss military cooperation in Syria.
White House spokesman Josh Earnest called the Russian offer a sign of "desperation."
"We're not interested in doing that, as long as Russia is not willing to make a constructive contribution to our counter-[Islamic State] effort," Earnest said.
"Russia has their own agenda... It's not particularly surprising to me that President Putin would resort, in some desperation, to try to send the second-highest ranking official in the Russian government to the United States to try to convince us to join them."
Kremlin spokesman Dmitry Peskov expressed "regret" at the U.S. "refusal to start dialogue."
Putin on October 15 accused Washington of being "unconstructive" on Syria.