Russia has positioned seven tanks at a Syrian airfield where it has been building up defences, US officials claim.
One of the officials, who was speaking on condition of anonymity, said the Russian T-90 tanks and artillery have been seen at the airfield near the port city of Latakia, a stronghold of President Bashar al Assad.
It comes following news last week that Russia has sent military hardware and troops to bolster Syria's embattled national forces.
The equipment said to have been deployed by Moscow included two tank landing ships and extra aircraft, along with a small number of naval infantry forces.
While the aim of the Russian military build-up remains unclear, a US official said last week that the purpose may be to help rebuild or defend the airfield.
Moscow has confirmed it has "experts" on the ground, but it has refused to give further details about its military presence in the country.
Damascus has denied Russian forces were involved in combat, but a Syrian official said the number of Russian "experts" had increased in the past year.
However, Syria's ambassador to Moscow has said talk of Russian troops on the ground is "a lie".
Sky's Foreign Affairs editor Sam Kiley said: "This is consistent with a build-up of Russian forces in Syria.
"The Russians are staunch allies of the Damascus regime, but they have been helping to rebuild two significant airfields including this one outside Latakia.
"They are now defending them with these Russian-manned Russian tanks and there is also an infantry battalion of around 1,000 men that has been deployed in western Syria to defend the port of Tartus."
Last week David Cameron declared that "hard military force" was needed to reduce the number of refugees fleeing Syria.
Millions are fleeing the country "because Assad has butchered his own people and because ISIL [Islamic State] have in their own way butchered others," the Prime Minister told MPs.
"Assad has to go, ISIL has to go and some of that will require not just spending money, not just aid, not just diplomacy but will on occasion require hard military force," he added.
