"We will not only continue strikes, ... we will also increase their intensity," said Andrei Kartapolov from the Russian Army's General Staff, according to RIA news agency.

Russia's air campaign in Syria, where a U.S.-led air coalition and fighters on the ground from regional states are already entangled in the four-year-old civil war, has drawn strong criticism from the United States and its allies.

Kartapolov said Russia's Air Force had targeted more than 50 Islamic State objects in Syria in more than 60 flights over a three-day period.

He said the strikes had significantly reduced the military potential of terrorists in Syria and were mainly aimed at command centres, weapons stores and workshops where arms were made.

"Panic and desertion have started in their ranks. Around 600 mercenaries have left their positions and are trying to cross into Europe," he told Russian news agencies.

"The strikes were carried out around the clock from the Hmeymim air base along the whole depth of the territory of Syria," he said.

Russia had warned the United States ahead of launching its air strikes and recommended the United States stop its own flights in areas where Russia's Air Force was operating as well as remove all U.S.-funded specialists in those areas, he added.

"The Americans told us during discussions that no one apart from terrorists were in that region," Kartapolov said, referring to the area where Russia's Air Force was active, according to Interfax.