The Russian military sent fighter helicopters and jets to Qamishli airport in the region east of the Euphrates River in Syria's east, where American forces and their allies, the Kurdish Syrian Democratic Forces (SDF), are deployed.
The development took place amid reports that Turkey was preparing to launch an offensive in northeastern Syria.
Six Russian helicopters carried out surveillance flights along the Syrian-Turkish border on Saturday.
An informed military source from the SDF said the Syrian army and Russian forces have reinforced their positions in the cities of Hasakeh and Qamishli and nearby areas.
American forces, meanwhile, deployed patrols in northeastern Syria, inspected the border and listened to the locals' concerns over the possible Turkish operation.
Turkey's current and future military operations on its southern borders do not target its neighbors' sovereignty but are necessary for Turkish security, the country's National Security Council (MGK) said on Thursday.
The MGK statement followed President Recep Tayyip Erdogan's declaration on Monday that Ankara would soon launch new military operations on its southern borders to expand 30-km (20-mile) deep safe zones and combat what he described as terrorist threats there.
"Operations being carried out now and in the future to remove the terrorism threat on our southern borders do not target our neighbors' territorial integrity and sovereignty in any way," it said after a three-hour meeting chaired by Erdogan.
Any operations were expected to target northern Syria, where Turkey has launched several incursions since 2016, mainly targeting the US-backed Kurdish People's Protection Units (YPG).
Erdogan reiterated on Saturday that Turkey was not seeking to undermine the sovereignty of its neighbors, but it will also "not allow anyone to violate Turkish territories."
A YPG spokesman said Saturday the forces were taking Turkey's threats seriously and they were ready to defend their gains, forces and people. "We will resist to the end," he vowed.
He stressed that the forces have committed to the de-escalation agreements and ceasefire reached between Washington, Moscow and Ankara in 2019.
"We withdrew our forces to avoid a war, but are on alert for any attacks," he added.
Moreover, he stressed that any Turkish attack on the regions east of the Euphrates would not be possible without the agreement of international forces.
"We are in daily contact with Moscow and Washington, but we are relying on our forces instead of international ones," he remarked.
Russia and the US are guarantors of the de-escalation agreements with Turkey, so they should assume their responsibilities, he urged.
