Ashton Carter
Ashton CarterReuters

US Defense Secretary Ashton Carter said Thursday Russia will soon begin to suffer casualties after dramatically expanding its military support for long-time ally, Syrian President Bashar al-Assad.

"This will have consequences for Russia itself which is rightly fearful of attacks...in coming days, the Russians will begin to suffer from casualties," Carter said at a NATO defense ministers meeting in Brussels dominated by the Syrian crisis.

The warning comes after the US State Department said Wednesday that most of Russia's barrage of airstrikes in Syria are not hitting Islamic State (ISIS) terrorists as it claims.

Russia conducted its first airstrikes in Syria last week, claiming it had been targeting ISIS in the Homs and Hama districts of the country, but an unnamed American official later said the targets were US-backed "moderate" rebel groups fighting ISIS.

While the airstrike campaign began on September 30, on Wednesday Moscow ramped up its war by unleashing cruise missiles from the Caspian Sea.

It apparently seeks to win back territory from rebel groups for its ally Assad, and likewise Iran has been sending in hundreds of troops to help in the massive offensive.

AFP contributed to this report.