WASHINGTON — The United States yesterday (Sept 8) 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 US 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 Assad of Syria — divisions that are likely to be on display when President Barack Obama and President Vladimir V Putin speak to the UN General Assembly this month.

The administration’s concerns were fuelled 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 a US 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 US 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 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 yesterday that they had closed their nation’s airspace to Russian transport planes through Sept 24.

“The reason for the refusal is associated with incorrect information in the requests to fly over the territory of the Republic of Bulgaria regarding the purpose of the flights and the cargo,” Bulgaria’s foreign minister said in a statement.

While Bulgaria’s move will block one of the major air routes from Russia to Greece, it does not fully resolve the issue since Russia may still be able to continue its flights over Iran and Iraq. There is also a route from Russia to Syria over Turkey, but Russians avoid Turkish airspace because they fear the Turks will force a Russian military plane to land, as they have in the past.

State Department officials declined to discuss publicly what requests have been made to foreign governments to close their airspace to Russian aircraft, citing the need to maintain the confidentiality of diplomatic communications. But the officials made clear they were pressing the issue.

“We have encouraged our allies and partners to ask tough questions of Russia’s increased military deployments to Syria,” said Mr John Kirby, the State Department spokesman.

At the least, the flights have highlighted the deepening differences between the United States and Russia over Syria. While the Obama administration has argued that Assad’s brutal crackdown against his opponents fueled the sectarian passions that strengthened the Islamic State, the Russians still appear to see the Syrian president as a bulwark against extremists, at least for now.

Western officials say that Russia’s intentions are not entirely clear. Although Putin spoke in Vladivostok last week about the need to form a coalition against the Islamic State, Iran and the Syrian government appear to the only potential members so far.

One possibility is that Russia is not only trying to support the Syrian government but is trying to expand its role inside Syria so it can influence the choice of a new Syrian government in case Assad is ousted. Another theory is that Russia is putting itself in a position to defend a rump state should Assad be driven from Damascus and find refuge in a stronghold near the coast. None of these possibilities are mutually exclusive.

On Saturday, Secretary of State John Kerry called Sergey V. Lavrov, the Russian foreign minister and warned Russia not to expand its military role in Syria. On Monday, a spokeswoman for Lavrov said that the Kremlin had long provided military assistance to the Syrian government in its fight against extremists and expressed surprise at Kerry’s warning.

But Tuesday, Kirby repeated the criticism of Russia’s role.

“Russia is not a member of the coalition against ISIL, and what we’ve said is that their continued support to the Assad regime has actually fostered the growth of ISIL inside Syria and made the situation worse,” he said, using an acronym for the Islamic State. “If they want to be helpful against ISIL, the way to do it is to stop arming and assisting and supporting Bashar Al-Assad.” THE NEW YORK TIMES