The figure has doubled since Moscow began bombing the evil terror group in September, according to US officials.
They also claimed the number of bases used by Putin's killing machines in the war-torn nation has grown.
Hundreds of ISIS sickos are thought to have already been wiped out – along with the group's arms supplies.
A spokesman for the Russian foreign ministry refused to comment on the number of troops in Syria yesterday.
The US officials also claimed several Russians have been killed – but failed to give exact numbers.
The only death reported by the Russian government is a serviceman who committed suicide.
Washington has repeatedly slammed Putin's intervention in the Syria crisis.
Insiders have accused the Russian president of trying to prop up president Bashar al-Assad.
Putin hit back at their claims last month, saying his country is committed to fighting terrorism.
He added: "I think some of our partners simply have mush for brains.
"They do not have a clear understanding of what really happens in the country and what goals they are seeking to achieve."
The US is finally set to send special operations forces into Syria after more than a year of unsuccessful air strikes.
(The Daily Star)
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