The United States 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 build up 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, quoting a diplomatic source as saying that Russia was seeking permission to run the flights up to Sept. 24. Russia, which has a naval base 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.

Salih Mahmud Leyla | Anadolu Agency | Getty Images

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 Islamic State 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 Islamic State, 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 United States, Saudi Arabia and Turkey. The Syrian army and allied militia have lost significant amounts of territory to insurgents this year. Assad said in July the Syrian army faced a manpower problem. Russia has been trying to build a wide coalition including Damascus to fight Islamic State, which was reported on Monday to have captured an oil field from government forces near the city of Palmyra.

The external courtyard of the sanctuary of Baal in the ancient oasis city of Palmyra, Syria. Joseph Eid | AFP | Getty Images

But the idea has been rejected by enemies including the United States and Saudi Arabia, who see Assad as part of the problem.

A senior U.S. official told Reuters on Saturday that U.S. authorities have detected "worrisome preparatory steps," including transport of prefabricated housing units for hundreds of people to a Syrian airfield, that could signal that Russia is preparing to deploy heavy military assets there. The official, speaking on condition of anonymity, said Moscow's exact intentions remained unclear but that Kerry called Lavrov to leave no doubt about the U.S. position. A Syrian military official has said Syrian-Russian military relations have witnessed a "big shift" in recent weeks.

Enlarging runways in the North

The United States and Turkey are planning to open a new front against Islamic State in an area of northern Syria near the border with Turkey. They aim to drive the jihadists from the area with the help of rebels on the ground.

A Lebanese newspaper reported on Monday that Russian military experts who arrived in Syria weeks ago have been inspecting air bases and working to enlarge some runways, particularly in the north, though Moscow had yet to meet a Syrian request for attack helicopters. As-Safir, citing a Syrian source, said there had been "no fundamental change" in Russian forces on the ground in Syria, saying they were "still operating in the framework of experts, advisers, and trainers". As-Safir said the Russians had "started moving towards a qualitative initiative in the armament relationship for the first time since the start of the war on Syria, with a team of Russian experts beginning to inspect Syrian military airports weeks ago, and they are working to expand some of their runways, particularly in the north of Syria." The newspaper, which is well-connected in Damascus, said nothing had been decided about "the nature of the weapons that Damascus might receive, though the Syrians asked to be supplied with more than 20 Russian attack helicopters, of the Mi-28 type".