REUTERS - 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, a Lebanese newspaper reported.

As-Safir, citing a Syrian source, said on Monday 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".

Secretary of State John Kerry told his Russian counterpart on Saturday the United States was deeply concerned about reports that Moscow was moving toward a major military build-up in Syria widely seen as aimed at bolstering President Bashar al-Assad.

Russia has been a vital ally for Assad throughout the war that is estimated to have killed a quarter of a million people and fractured Syria into a patchwork of areas controlled by rival armed groups, including Islamic State, and the government.

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, added that nothing had been decided on "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".

The Syrian government, whose alliance with Moscow dates to the Cold War, has lost control of much of northern Syria to groups including Islamic State. The United States, already leading a coalition in a bombing campaign against Islamic State in Syria, is planning with Turkey to open a new front against the jihadist group in northern Syria near the Turkish border.

The United States and governments including Saudi Arabia and Turkey want to see Assad, also an ally of Iran, gone from power.

A Syrian military official declined to comment on the details of As-Safir report. The official however reiterated previous comments that the Russian-Syrian military relations had witnessed a big shift in recent weeks.

The official said the Russian shift was prompted by the danger represented by Islamic State and other groups fighting the Syrian state.

"It is obvious that the Russians will be more resolute in dealing with this situation. This forms a danger to the allies of Russia," the official said.

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 readying deployment of heavy military assets there.

The official, speaking on condition of anonymity, said Moscow's exact intentions remained unclear but that Kerry called Russian Foreign Minister Sergei Lavrov to leave no doubt about the U.S. position.