(CNN) Russia is intensifying its airstrikes in Syria, which it says have "considerably reduced" the combat potential of militants.
The Russian air force began strikes in Syria on Wednesday. In the past 24 hours, it has carried out 20 flights targeting 10 ISIS positions in Idlib province, the Russian Defense Ministry announced in a statement Sunday.
Since 2011, Syria has been torn apart by a brutal civil war that has seen the rise of extremist groups including ISIS. Moscow supports the regime of President Bashar al-Assad and says it is targeting ISIS, al Nusra and "other terrorist groups."
But the U.S-backed coalition against ISIS -- comprising the United States, Britain, Turkey, France, Germany, Qatar and Saudi Arabia -- accuses Russia of attacking civilians and Syrian groups opposing al-Assad.
British Prime Minister David Cameron, in an interview with the BBC on Sunday, said most of the strikes appeared to have targeted areas not under ISIS control.
"What is happening is that they are backing the butcher Assad, which is a terrible mistake for them and for the world. It's going to make the region more unstable, it will lead to further radicalization and increase terrorism," he said.
Al-Assad told Iranian television station Khabar TV he was optimistic that a coalition of Iran, Russia, Iraq and Syria would be successful, according to a transcript carried by Syrian state news agency SANA.
"It must succeed; otherwise the whole region, not only one or two countries, will be destroyed," he said.
"When these countries unite against terrorism and fight it militarily and in the areas of security and information, in addition to other aspects, this coalition will, no doubt, achieve real results on the ground, particularly that it enjoys international support from countries which do not have a direct role in these crises and in this region," al-Assad said.
In a speech Saturday, Russian Col. Gen. Andrei Kartapolov announced that Russia was intensifying its strikes. He said that before Russia began air operations, "American experts informed us that there was nobody but terrorists in the district."
Russian intelligence suggested militants had begun fleeing the strikes, which had targeted ISIS command centers, ammunition depots and training camps, Kartapolov said, according to Russia's Ministry of Defense.
"Panic and desertion has started among them. About 600 mercenaries have left their positions and are trying to flee to Europe. Thus, the Russian airstrikes will not only be continued; their intensiveness will be increased," he said.
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On Saturday, Russia said its soldiers had bombed nine ISIS positions in Raqqa, the group's de facto capital. Tactical bombers destroyed various militant targets, including ammunition and oil depots, and all-terrain vehicles, it said in a statement.
Opposition groups alleged at least 11 people had been killed in a double strike by Russia on the village of Ehsim in Idlib province. CNN could not independently verify it was a Russian strike.
Syria and the U.S.-backed coalition against ISIS also have continued strikes.