"Over almost 15 months, a US-led coalition has been carrying out its operation in the Iraqi territory and in the Syrian airspace," Peskov said in an interview aired by Rossiya-24 TV channel.
"Unfortunately, this operation, which has been underway for over a year, has led to the expansion of ISIL-controlled territory."
Peskov added that a "fairly short-term" involvement by the Russian Aerospace Forces has "reversed the trend."
"The terrorist-held territory has begun to shrink," Peskov observed, adding this proved Russia’s operation was more effective.
Russia Ready for Cooperation With US-Led Coalition on Syria
Russia is ready for any level of cooperation on the Syrian issue with the US-led coalition, but has not received reciprocity on the matter yet, the spokesman said.
"Well, the fact that Russia is offering to exchange targets and exchange information on various groups… is well known. The Russian side holds the door open for any level of cooperation for which the US-led coalition would be ready. Let’s say we don’t see the reciprocal readiness yet."
A US-led international coalition has been conducting airstrikes against ISIL in Iraq and Syria since September 2014. Russia started a separate air campaign against ISIL in Syria on September 30, 2015, at the request of Syrian President Bashar Assad.