Russian Build-Up Continues at Base in Syria, Causing Concern Among US Officials

— -- Russia continues to transport equipment to an airbase in the Syrian port city of Latakia -- a move that U.S. officials suggest could mean it is being prepared to handle significant air activity.

Four large Russian transport aircraft arrived over the weekend at the base with unspecified equipment, and two tank landing ships have also arrived at the Russian naval base in Tartus, south of Latakia, American officials said, noting it remains uncertain what Russian plans are in developing what appears to be a new air hub in Latakia.

The U.S. started seeing the arrival of Russian equipment last week at a Syrian air base co-located at the Bassel Al Assad International Airport in Latakia, a city northwestern Syria that is a stronghold of support for embattled Syrian President Bashar Al Assad, U.S. officials said.

What initially drew the concern of American officials was the arrival at the airfield of temporary modular building structures that could be used to house hundreds of personnel. Construction has yet to begin on those structures. The Russians were also spotted establishing a temporary air traffic control system, U.S. officials said.

Secretary of State John Kerry called Russian Foreign Minister Sergei Lavrov on Saturday to express American concerns about the build-up, which "could further escalate the conflict" in Syria, the State Department said in a statement announcing Kerry's call. Kerry called Lavrov again today about the issue, U.S. officials said.

But the flow of Russian materials continued this weekend with the arrival in Latakia of four massive Antonov AN-124 “Condor” cargo aircraft carrying undetermined cargo, U.S. officials said.

A Russian Ilyushin-62 passenger aircraft also arrived at the airport carrying what are believed to be a small number of Russian military personnel. One U.S. official described naval infantry personnel that may be involved in force protection at the base, another U.S. official said they numbered 40 personnel and that it was unclear if they are serving as security or as an initial wave of Russian advisers.

So far, the activity at the base has only involved the cargo and passenger aircraft -- no Russian fighter aircraft or drones have been spotted as had been reported in early media reports.

In recent days, two Russian navy tank landing ships or LST’s (Landing Ship, Tanks) carrying undetermined cargo have also arrived at the Russian naval base at Tartus, located 50 miles south of Latakia, U.S. officials said. Russia has had a naval base in Tartus since the early 1970's.

Russian Foreign Ministry spokeswoman Maria Zakharova told several news outlets today that Russia has supplied the Assad regime with military equipment and sent "military specialists" to train Syrian forces in that equipment.

"Russia has never made a secret of its military-technical cooperation with Syria," she told the Associated Press. She confirmed and "repeat once again that Russian military specialists are in Syria to help them master the weapons being supplied."

A U.S. official said the low number of Russian personnel now in Syria is not the main U.S. concern right now, but rather the continuing flow and capability of Russian equipment through what was described as a "significant" air expeditionary capability

Pentagon Press Secretary Peter Cook said on Tuesday that a broader Russian effort "to bolster the Assad regime right now would potentially be destabilizing." He said of Russian plans, "we don't know exactly what they're doing."

Given that uncertainty, there is a wide range of speculation by American intelligence about Russian intentions at the base in Latakia, U.S. officials said.

"Are they planning to support the Assad regime by going after rebel forces, provide advisers, boost Syria’s air defense systems or send in fighter aircraft to strike at rebel forces?" said one official.

Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, told ABC News' Martha Raddatz last week that a Russian effort to train and equip Syria’s military “would be one thing,” but “if they actually participate in the campaign against, what they believe to be ISIL, it could complicate things.” ISIL is another name used to describe ISIS.

“I don’t think they would be as discriminating among groups as maybe we would,” Dempsey said. “I mean there’s probably four, five, six, 10, 15 groups, and if they’re all declared to be anti-regime, and then the weight of this effort would be thrown against them, then I think that’s a problem.”

Originally, Russian aircraft were flying routes over Bulgaria and Greece towards the Mediterranean to fly into Latakia. But Bulgaria has now denied overflight access to what Russian officials have characterized as humanitarian assistance. Since then, Russian flights are now flying an easterly route over the Caspian Sea then flying over Iran and Iraq.

Russian news agencies quote a Russian embassy official in Tehran as saying Iran has given permission for Russian overflights through its airspace.