Meanwhile, Syria ground troops have launched an offensive under Russian air cover.
The Defence Minister Sergei Shoigu said four warships fired 26 sea-based cruise missiles at 11 targets, destroying them and causing no civilian casualties.
Russia denies claims that its week of strikes have mainly hit non-IS targets.
The Syrian Observatory for Human Rights has reported that “this is the most intense fighting in months” in Hama and Idlib provinces. The clashes followed a wave of Russian air strikes in the same areas, it said.
It, however appears to be the first co-ordinated offensive attack since the air campaign by Russia – a staunch ally of Syrian President Bashar al-Assad – began on 30 September.
Russia says it is targeting “all terrorists”, but at least some of its air strikes have reportedly hit civilians and Western-backed rebels.
In a separate development, Russia’s foreign ministry has said Moscow is willing to establish contact with a Western-backed rebel group, the Free Syrian Army, to discuss fighting IS “and other terrorist groups”, and “preparing the ground for a political settlement in Syria“.
US Defence Secretary Ash Carter said that coalition forces fighting IS in Syria would not co-operate with Russia.
“We believe Russia has the wrong strategy,” he said. “They continue to hit targets that are not IS. We believe this is a fundamental mistake.”
Pentagon officials later revealed they had had to carry out at least one “safe separation” manoeuvre to avoid a US jet coming too close to a Russian aircraft over Syria. They said this happened after 1 October, without giving a specific date.