Russia, which has a naval maintenance facility in the Syrian port of Tartous, has sent regular flights to Latakia, which it has also used to bring home Russian nationals who want to leave.

The U.S. has asked Greece to deny Russia the use of its airspace for supply flights to Syria, a Greek official said on Monday, after Washington told Moscow it was deeply concerned by reports of a Russian military buildup in Syria.

The Greek foreign ministry said the request was being examined. Russian newswire RIA Novosti earlier said Greece had refused the U.S. request, adding that Russia was seeking permission to run the flights up to September 24.

Kremlin spokesman Dmitry Peskov said Moscow would not give any official reaction until there was a decision from Athens.

Russia, which has a naval maintenance facility in the Syrian port of Tartous, has sent regular flights to Latakia, which it has also used to bring home Russian nationals who want to leave.

U.S. Secretary of State John Kerry told his Russian counterpart Sergei Lavrov on Saturday that if reports of the build-up were accurate, that could further escalate the war and risk confrontation with the U.S.-led alliance that is bombing ISIS in Syria.

Lavrov told Kerry it was premature to talk about Russia’s participation in military operations in Syria, a Russian foreign ministry spokeswoman told RIA Novosti on Monday.

Lavrov confirmed Russia had always provided supplies of military equipment to Syria, saying Moscow “has never concealed that it delivers military equipment to official Syrian authorities with the aim of combating terrorism.”

Russia has been a vital ally of President Bashar al-Assad throughout the war that has fractured Syria into a patchwork of areas controlled by rival armed groups, including ISIS, leaving the government in control of much of the west.

Foreign states are already deeply involved in the war that has killed a quarter of a million people. While Russia and Iran have backed Assad, rebel groups seeking to oust him have received support from governments including the U.S., Saudi Arabia and Turkey.

The Syrian army and allied rebels have lost significant amounts of territory to insurgents this year. Assad said in July the Syrian army faced a manpower problem.