The air base, which used to belong to the Syrian Army, is now under control of the US-backed and Kurdish-led so-called Syrian Democratic Forces (SDF) who captured the base from the so-called "moderate rebels" last year, while receiving support from the Russian Air Force.
According to the sources close to the Russian military, members of the Russian and Turkish delegations were on board the helicopters.
The two delegations are said to have met with the Kurdish military leaders at the base.
This comes days after the Syrian Democratic Forces announced full liberation of the northern Syrian city of Raqqah, the former de-facto capital of ISIS.
Comment: The Americans are clearly banking on the SDF to hold territory and act as a thorn in the side of the Syrian government. But it remains to be seen what future actions the SDF will actually take. They may be more willing to work with the Russians and Assad's government than the Americans hope.