Russian warplanes mounted airstrikes against targets in Syria for a second day on Thursday, opposition groups said, adding that the raids targeted rebels who have fought both Syrian government forces and Islamic State militants.
Russia's Defence Ministry said its jets had mounted 12 strikes against Islamic State targets in Syria in the past 24 hours.
Samir al-Nashar, a member of the Western-backed Syrian opposition National Coalition, told dpa that the strikes hit the town of Jisr al-Shoghur in the north-western province of Idlib and the central province of Hama.
They targeted posts belonging to Jaish al-Fateh (Army of Conquest), a rebel alliance that includes al-Nusra Front - a branch of the terrorist organization al-Qaeda - and opposes Islamic State.
Jaish al-Fateh has fought fierce battles against Islamic State in some areas in Syria and has also made territorial gains against the army of the Syrian regime in Idlib in recent months.
Kremlin spokesman Dmitry Peskov said the airstrikes were targeting Islamic State and other terrorist and extremists groups in collaboration with the Syrian military, state news agency TASS reported.
The Russian Defence Ministry's spokesman, General Igor Konashenkov, said an Islamic State command facility had been destroyed in the area of Latamna in the Hama province.
More than 50 Russian warplanes and helicopters are part of the group conducting airstrikes on Islamic State targets in Syria, Konashenkov said, according to the Interfax news agency.
"During the night, four airstrikes against Islamic State targets were conducted" using Su-24M and Su-25 attack aircraft, he told a news conference.
Russia has also deployed a battalion of marines to guard an airbase used by the Russian air force near the Syrian port city of Latakia, Konashenkov said.
Syrian state media and opposition representatives have reported Russian strikes in areas of central Syria, including Hama, where other rebel forces including the Syrian branch of al-Qaeda - but not Islamic State - are typically thought to operate.
The head of the Syrian Observatory for Human Rights, Rami Abdel-Rahman, confirmed the latest strikes, saying he believed they had been launched by Russian jets.
The pro-Syrian Lebanese television broadcaster Al Mayadeen reported that at least 30 Russian air raids had been carried out on the outskirts of Jisr al-Shoghur and Hama.
Russian President Vladimir Putin said in televised comments that initial reports of civilian deaths due to the Russian airstrikes appeared before the strikes started.
He denounced such reports as part of a media campaign against Moscow, echoing similar comments by the Foreign Ministry's spokeswoman the previous day.
On Wednesday, Russia began its first airstrikes in Syria, saying it targeted and destroyed Islamic State positions.
But the moderate Syrian opposition said the strikes had targeted territory under its control and killed civilians.
The United States has expressed doubts that the Russian strikes are solely targeting Islamic State militants.
By Weedah Hamzah and Peter Spinella