A US newspaper has warned of "an escalation of Russia's military engagement in Syria's civil war.

Los Angeles Times cited evidence captured by US intelligence officials.

The evidence included "satellite images of an apparent Russian base for staging troops and heavy equipment under construction near a port city that is a stronghold for Syrian President Bashar Assad," the paper quoted the US officials as saying.

"The reconnaissance photos of possible military housing being built near the international airport in Latakia province provides strong evidence of deepening involvement by President Vladimir Putin's government in the four-year-old Syrian war," according to the officials, who, the paer said, requested anonymity because they were not authorized to discuss the matter publicly.

"In addition, the Russians have asked at least one country bordering Syria for a window of time to fly a detachment of warplanes over its territory and into Syria," said the officials.

They did not name the country.

The paper said in the report that the White House said it was closely monitoring reports of Russian military operations in Syria after images of Russian-built warplanes and reconnaissance drones appeared on social media accounts belonging to Syrian opposition fighters.

"The pictures purportedly showed the aircraft operating near Idlib province in northwestern Syria. Idlib, like Latakia, is in the part of the country dominated by the Alawite sect, the religious minority group to which the Assad family belongs," said the paper.