IHS Jane's said Tuesday that it had spotted two previously unreported sites in satellite imagery of western Syria where steps appear to be being taken to receive Russian forces.
The U.S. government, which has been monitoring with concern the influx of Russian troops and military hardware into Syria, said last week that Russia appeared to be setting up a base
near Latakia, a port city on Syria's Mediterranean coast.
Moscow is a staunch ally of Syrian President Bashar al-Assad, whom the United States has repeatedly called on to step down to help bring about an end to the bloody civil war.
The two new sites where IHS Jane's analysts say Russian troops could go are the the Istamo weapons storage complex and the Al-Sanobar military complex, both north of the Latakia base.
"Military personnel would likely arrive at these two bases as part of the rapid expeditionary force buildup currently centered around Latakia," the analysts said.
Asked about the Jane's report at a regular news briefing Tuesday, Pentagon spokesman Peter Cook declined to comment on "intelligence matters."
"Just rest assured that we're keeping a very close eye on what we see moving in there," he told reporters, adding that Pentagon officials believe "there may be an opportunity for the Russians to play a constructive role" in the fight against ISIS.
The Jane's analysts said they had observed "a substantial increase" in Russian combat jets near Latakia, counting four Su-30SM multirole combat aircraft, 12 Su-25 ground attack aircraft, 12 Su-24M attack fighters.
That chimes with an assessment provided to CNN by a U.S. official earlier this week that Russia has more than 25 fighter and attack aircraft, 15 helicopters, nine tanks, three surface-to-air missile systems and at least 500 personnel on the ground in Syria.
Russian aims remain unclear
Russia has also started to fly unmanned aircraft
over Syria in what appear to be surveillance operations, two U.S. officials told CNN.
The Russian military moves amid the volatile Syrian conflict have fueled fears of a miscalculation that could further escalate hostilities. A U.S.-led coalition is carrying out an aerial bombing campaign against ISIS positions in Syria and Iraq.
Despite repeated contacts on the subject by the senior government officials, Cook said U.S. officials "still do not have an entirely clear picture of exactly what the Russians are hoping to do in Syria."
Moscow says its fight is with ISIS, the brutal Islamist group that's captured large swathes of territory. But with shifting enemies and alliances, the Syrian conflict is anything but straightforward.
Retired Gen. David Petraeus, the former commander of U.S. forces in both Iraq and Afghanistan, said Tuesday that he believed Moscow was trying to shore up the al-Assad regime
in order to preserve Russian influence in the Middle East.
"Russia's recent military escalation in Syria is a further reminder that when the U.S. does not take the initiative, others will fill the vacuum, often in ways that are harmful to our interest," Petraeus said before the Senate Armed Services Committee.