Moscow has come under increased international pressure in recent days to explain its moves in Syria, where the Kremlin has been supporting Syrian President Bashar al-Assad in a 4-1/2-year war.

The Pentagon declined to directly comment on the Reuters report, saying it could not discuss U.S. intelligence. But a U.S. Defense Department spokesman said recent actions by Moscow suggested plans to establish a forward air operating base.

"We have seen movement of people and things that would indicate that they plan to use that base there, south of Latakia, as a forward air operating base," Pentagon spokesman Captain Jeff Davis told a news briefing.