The Syrian air force has been reportedly carrying out airstrikes on Islamic State militants using newly arrived Russian aircraft.
Media reports quoting local sources say the strikes have taken place in the northern part of the country.
The Syrian military has been increasing airstrikes on Islamic State positions in recent days, amid a Russian military buildup in the country believed to involve fighter jets, helicopters and personnel on the ground, among other military supplies.
Earlier this week, U.S. officials told VOA that Russia has 28 advanced attack aircraft at an air base near the Syrian coastal town of Latakia, up from four confirmed Russian warplanes last week.
U.S. Secretary of State John Kerry said U.S. military experts believe the Russian planes sent to the Syrian air base are there to protect Russian soldiers and assets already on the ground.
Kerry warned that Russian actions in Syria risk a confrontation with the coalition opposing the Islamic State group, while Russia said the U.S. is becoming more receptive to Russia's position on the conflict.
Signs have emerged that Russia may be preparing to expand its military presence in Syria.
Satellite imagery released Tuesday by the defense research firm IHS Jane's show new construction at two other Syrian military facilities near Latakia.
Jane's analysts said the presence of Russian-style military tents at one of the sites suggested preparations for a larger force.