WASHINGTON — The United States on Tuesday moved to head off preparations for a suspected Russian military buildup in Syria as Bulgaria agreed to an appeal from the Obama administration to shut its airspace to Russian transport planes. The planes’ destination was the Syrian port city of Latakia.
The administration has also asked Greece to close its airspace to the Russian flights, Greek and American officials said, but Greece has not publicly responded to the request.
The apparent Russian military preparations and the Obama administration’s attempt to block them have escalated long-running tensions between the White House and the Kremlin. Although the United States and Russia agree that the Islamic State is a threat, the new dispute shows that they remain far apart on how best to combat the militant group and on the political future of President Bashar al-Assad of Syria — divisions that are likely to be on display when President Obama and President Vladimir V. Putin speak to the United Nations General Assembly this month.
The administration’s concerns were fueled last week by intelligence reports indicating that Russia appeared to be making preparations to deploy advisers and military personnel to an airfield south of Latakia and might also bring in aircraft and fly airstrikes from there. Those preparations included the delivery of prefabricated housing for as many as 1,000 personnel and a portable air traffic control station to the airfield.
Over the weekend, two giant Russian Condor transport planes ferried more supplies and equipment from an air base in southern Russia across Iran and Iraq to Latakia, according to an American official who spoke on the condition of anonymity because the official was discussing intelligence reports.
A Russian troop transport plane, probably an Ilyushin model, also landed at the same airfield in Latakia over the weekend. That aircraft, which flew over Greece and Bulgaria, is believed to have carried Russian military personnel.
“They’re clearly establishing some sort of forward operating base,” the American official said. There have also been unconfirmed sightings of Russian Spetsnaz special forces at the Syrian Naval Academy, officials said.
Providing a benign explanation for the operations, the Russian news media has suggested that the planes were carrying humanitarian assistance. That is the same rationale Russia used to explain convoys that are believed to have delivered military supplies to Ukrainian separatists and that Iran has used to fly arms to Damascus to support the Assad government.
Bulgarian officials said on Tuesday that they had closed their nation’s airspace to Russian transport planes through Sept. 24.