Russia has started flying drones over Syria in what appears to be its first aerial surveillance operations over the war-ravaged country, U.S. defense officials told NBC News.
Although the officials said there was no indication Russia had yet conducted combat missions using fighter jets and attack helicopters, they said they expected such operations to start soon.
Moscow has already sent in four SUKHOI attack fighter jets, four HIND attack assault helicopters, four HIP transport helicopters, along with tanks and artillery, the officials said.

Image: A MI-8 Helicopter flies during Russias large-scale Center-2015 military exercises
A MI-8 Helicopter flies during Russias large-scale Center-2015 military exercises at Donguzsky Range on Sept. 19, 2015 in Orenburg, Russia. The exercises aim to contain the outbreak of an armed conflict in Central Asia. Sasha Mordovets / Getty Image
In addition, it has now flown in large quantities of air-to-ground missiles, high-caliber ammunition for attack-jet machine guns and aircraft fuel, the officials added.
According to one senior official: "They didn't fly all those attack aircraft in there to leave them sit on the runway."
The Russian Ministry of Defense was not available for comment when contacted by NBC News.
Russia, which supports the regime of Syrian President Bashar Assad, has over the past two weeks been building a military presence near the Syrian port city of Latakia.
The U.S. supports rebels fighting Assad, but both Washington and Moscow have a common enemy in ISIS, which has more recently joined the conflict and been the target of U.S.-led airstrikes.
The U.S. officials said it is still unclear exactly what the Russians intend to do militarily, but there is growing concern they could reshape and control much of the combat operations against ISIS in Syria.
One senior official said: "They could take over control and there's nothing we could do about it."
In a 50-minute phone call on Friday, U.S. Defense Secretary Ashton Carter expressed concern about the military buildup to Russian Defense Minister Sergei Soichu. Carter said the U.S. and Russia needed to "deconflict" air operations in Syria to "avoid any mistakes or miscalculations" between the two forces.