DAMASCUS - Photos allegedly showing Russian soldiers, reports of military deliveries and overflight requests are swelling fears among the United States and its allies that Russia is covertly bolstering the Syrian regime.
Moscow, an ally of Syria's President Bashar al-Assad, has had a military presence in the country since the Soviet era.
Though traditionally confined to a small naval facility in the port of Tartus, Russian infrastructure is spreading, US officials say.
Housing units have been set up that can accommodate "hundreds of soldiers" as well as a portable air traffic control station near the Syrian port city of Latakia, US officials said on condition of anonymity.
Aerial imagery showed Russia was focusing on Bassel al-Assad International Airport, south of Latakia on Syria's Mediterranean coast, and on the Russian naval facility in Tartus.
Two tank-landing ships have recently arrived at Tartus and about a dozen Russian armoured personnel carriers are now at the airport, one US official said.
Moscow argues it has sent military equipment to Syria according to commercial contracts and that its specialists merely help train the Syrian army.
But recent reports suggest the Kremlin has more at stake as it pushes for a broader grouping to fight the Islamic State group.
Moscow has excluded Russian boots on the ground or help in air strikes, but provides military support to the Syrian army as one of Assad's few remaining allies to help "fight terrorism".
The Russians have recently hosted various meetings of Syria's disparate opposition groups, who disagree on the fate of Assad.
Assad said in March he would support more Russian military support in Tartus and other ports.
- Western concern -
Reports of a beefed-up Russian military presence come with Russian-Western relations in a post-Cold War nadir over the Ukraine crisis.
Moscow's initial denial that it had sent troops to the Ukrainian peninsula of Crimea ahead of its annexation in March 2014 casts a shadow on Russia's claims about its external military presence.
The New York Times reported Saturday that Russia had deployed an advance military team and delivered equipment to construct an air base near Latakia, an Assad regime coastal stronghold.
American officials also said under condition of anonymity at least three Russian military transportation aircraft -- two Antonov 124 Condor cargo planes and a passenger jet -- have landed at Latakia airport in recent days.
"All of this seems to be suggesting that Russia is planning to do some sort of forward air-operating hub out of this airfield," one official said.
US Secretary of State John Kerry called his Russian counterpart Sergei Lavrov to warn the conflict could escalate if Moscow does increase its military support, a fear echoed by NATO Secretary General Jens Stoltenberg.
White House deputy press secretary Eric Schultz on Wednesday said the United States would welcome any Russian contributions to the fight against the Islamic State group.
But "it would be unconscionable for any party, including the Russians, to provide any support to the Assad regime", Schultz told reporters.
- Incomplete evidence -
Damascus, like Moscow, has denied reports of increased Russian presence, accusing Western and Arab states of spreading misinformation to portray Syria as weak.
Independent political analyst Alexander Golts told AFP it was "normal" to combine arms deliveries with military advisor visits.
He said the pictures of Russian soldiers claiming to be in Syria were in fact of soldiers monitoring the Russian naval facility in Tartus.
"They [the military personnel] are there to ensure that the transfer of military equipment to Tartus goes smoothly," Golts said.
Some three dozen pictures allegedly showing Russian soldiers in Syria emerged in Russian media, heightening the speculation.
But one soldier who had tagged a picture of himself in Damascus on Russian social network VK said he was in fact a Belorussian national who set the location to the Syrian capital "just for fun" for a picture taken in Belarus.
Golts added there was no tangible evidence of Russian military presence in Syria outside Tartus but said "something might be going on that is worrying American intelligence".
Analyst Jeffrey White of the Washington Institute for Near East Policy said that "lots of Russian materiel" -- including combat and logistical type vehicles -- had been flowing through the Turkish Straits.
"I believe the primary purpose is to shore up the regime," White said. "It looks like maybe this started as early as April and just went undetected."
White said it could jeopardise the insurgency's chances of success, including in Latakia, a possible relocation area for Assad and his clan, which the regime's opponents have been targeting for months.