U.S. security officials told Reuters on Thursday that Russia has doubled its military personnel in Syria in its campaign to support Syrian President Bashar al-Assad.

On September 30 when Russia began its recent wave of airstrikes in Syria, there was an estimated 2,000 personnel. Now, they have not only doubled their people, but have also approximately doubled the number of bases they are using. Security officials stated that an exact number of personnel cannot be known since the Kremlin has suffered combat casualties in Syria.

Russian officials have declined to comment regarding the size of their military forces in Syria and did not answer any questions that were sent by Reuters.

However, they did state that no Russian troops have been in combat in Syria, although there are advisers and trainers working with the Syrian military. Reuters also reports that Russian forces are guarding the bases in western Syria.

The Kremlin further states that since they have been in Syria, they have only lost one soldier who committed suicide. The parents of the serviceman have stated they doubt this claim.

The U.S. has criticized Russia over their recent involvement in the Syrian civil war. While Russia has stated numerous times that they are there to fight the Islamic State, numerous reports have indicated that Syrian rebels have been the main targets of their airstrikes in order to support Syrian President Bashar al-Assad. The U.S. believes that Assad should step down from his leadership role in order for the Syrian civil war to end.