Speaking on condition of anonymity, the US official told AFP the seven T-90 tanks arrived in recent days but had not been sent outside the airbase near Latakia, on Syria`s Mediterranean coast.
And the artillery, which arrived last week, appears to have been sent there to protect the facility. There was no indication Russia had sent fighter jets or helicopter gunships to Syria.
"It appears, and all the indications are pointing, that (the artillery is) for airfield defense," the official said.
Another US official, also speaking on condition of anonymity, said Russia has installed enough modular housing units to house about 1,500 people.
It is not easy to gauge how many Russian troops are already on the ground, but the official said the number is in the "hundreds."
Russian help for Assad could seriously complicate the US-led coalition`s air strikes against Islamic State jihadists in Syria, with defense officials worried about the possibility of accidents if coalition and Russian planes operate in the same airspace.
Two Russian transport planes, purportedly carrying humanitarian aid, landed in Syria on Saturday, Russian state media said, and Russian Foreign Minister Sergei Lavrov last week said Moscow was sending military equipment along with aid "in accordance with current contracts."
At a news briefing, Pentagon spokesman Captain Jeff Davis said Russia apparently was establishing a forward air operations base in Latakia.
"It`s been a continued steady flow (of equipment) for the last week and a half or so," he said.
AFP reported last week that dozens of Russian naval infantry had arrived in Syria, along with two tank-landing ships and about a dozen Russian armored personnel carriers.
Russia is a staunch ally of the regime in Damascus and maintains a naval facility in Tartus province.
"We would welcome Russian contributions to the overall global effort against (the IS group) but things that continue to support the Assad regime, particularly military things, are unhelpful and risk adding greater instability to an already unstable situation," Davis said.
More than 240,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.
The government has lost large swathes of territory to rebels and jihadists such as the Islamic State group.
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