Russia has positioned about a half dozen tanks at an airfield at the centre of a military build-up in Syria, according to two US officials.
The officials said on Monday in Washington DC that the intentions of Russia's latest deployment of heavy military equipment to President Bashar al-Assad's army were unclear.
Russia has come under increased international pressure in recent days to explain its moves in Syria.
The Pentagon declined to directly comment on the Reuters report, saying it could not discuss US intelligence.
But a US defence department spokesman said recent actions by Russia suggested plans to establish a forward air operating base.
"We have seen movement of people and things that would indicate that they plan to use that base there, south of Latakia, as a forward air operating base," Captain Jeff Davis, a Pentagon spokesperson, said.
One of the US officials, who spoke to Reuters on condition of anonymity to discuss intelligence matters, said seven Russian T-90 tanks had been observed at the airfield near Latakia, an Assad stronghold.
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The two US officials said Russia had also stationed artillery that appeared to be arrayed defensively to protect Russian personnel stationed there.
Reuters has previously reported that Russia had deployed about 200 naval infantry soldiers to the airfield, as well as temporary housing units, a portable air traffic control station and components for an air defence system.
In a sign of the pace of Russia's build-up, Russia has been sending about two military cargo flights a day to the airfield over the past week, US officials say.
Russia has said it will continue providing military supplies to Syria and that its assistance to the Syrian army is in line with international law.
The US is using Syrian airspace to lead a campaign of air strikes against the Islamic State of Iraq and the Levant (ISIL) group.
So far, Russia has not sent combat aircraft or helicopter gunships to the airfield, the Pentagon said.
Both Russia and the US say their enemy is ISIL, whose fighters control large parts of Syria and Iraq.
But Russia supports the government of Assad in Syria, while the US says his presence makes the situation worse.
On the ground, Syrian state TV said on Monday that at least 20 people were killed in two car-bomb attacks in the northeastern city of Hasakah.
The broadcaster said they were killed in a blast in the Mahata neighbourhood of the city that followed a first blast in Khashman district.
State news agency SANA gave different tolls for the blasts, which it said were suicide car bombs.
The agency said five people were killed in Khashman and 12 in Mahata, and also reported that at least 70 others had been wounded.
A woman and her two children were among the dead, the Associated Press news agency reported.
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The UK-based Syrian Observatory for Human Rights also said the two explosions were suicide car bombs, although it could not immediately confirm a toll.
Rami Abdel Rahman, head of the Syrian Observatory, said the first blast hit a checkpoint belonging to the Kurdish security forces in Khashman, while the second struck the headquarters of a pro-government unit in Mahata.
Control of Hasakah city, and other parts of the province by the same name, is divided between Kurdish groups and forces loyal to Assad.
The city has regularly been targeted by ISIL, which controls some territory in Hasakah province.
ISIL entered the city and seized several neighbourhoods in June, but was expelled a month later after battles involving both government troops and Kurdish fighters.
The Syrian civil war, in which about 250,000 people have died, has caused nearly half of Syria's prewar population of 23 million to flee, with many thousands attempting to reach Europe.