Russian forces have begun participating in military operations in Syria in support of government troops, three Lebanese sources familiar with the political and military situation there said on Wednesday.The sources, speaking to Reuters on condition they not be identified, gave the most forthright account yet from the region of what US officials say appears to be a new military buildup by Moscow, one of President Bashar Assad's main allies, though one of the sources said the numbers of Russians involved so far were small.Two US officials said Russia has sent two tank landing ships and additional aircraft to Syria in the past day or so and has deployed a small number of naval infantry forces.The US officials, who also spoke on condition of anonymity, said the intent of Russia's military moves in Syria remained unclear. One of the officials said initial indications suggested the focus was on preparing an airfield near the port city of Latakia, an Assad stronghold.The moves come at a time when forces of Assad's government have faced major setbacks on the battlefield in a four-year-old multi-sided civil war that has killed 250,000 people and driven half of Syria's 23 million people from their homes.Syrian troops pulled out of a major air base on Wednesday, and a monitoring group said this meant government soldiers were no longer present at all in Idlib province, most of which slipped from government control earlier this year.Moscow confirmed it had "experts" on the ground.But Russia has declined to comment on the exact scale and scope of its military presence in Syria. Damascus denied Russians were involved in combat, but a Syrian official said the presence of experts had increased in the past year.Officials in the United States, which is fighting an air war against the Islamist militant group Islamic State in Syria and also opposes Assad's government, have said in recent days that they suspect Russia is reinforcing to aid Assad.Washington has put pressure on countries nearby to deny their air space to Russian flights, a move Moscow denounced on Wednesday as "international boorishness".Moscow's only naval base in the Mediterranean is at Tartous on the Syrian coast in territory held by Assad, and keeping it secure would be an important strategic objective for the Kremlin.Two of the Lebanese sources said the Russians were establishing two bases in Syria, one near the coast and one further inland which would be an operations base."The Russians are no longer just advisers," one of them said. "The Russians have decided to join the war against terrorism."Another of the Lebanese sources said that so far any Russian combat role was still small: "They have started in small numbers, but the bigger force did not yet take part ... There are numbers of Russians taking part in Syria but they did not yet join the fight against terrorism strongly."The Syrian official said: "Russian experts are always present but in the last year they have been present to a greater degree."Reflecting Western concern, Germany's foreign minister warned Russia against increased military intervention in Syria, saying the Iran nuclear deal and new UN initiatives offered a starting point for a political solution to the conflict.NATO Secretary General Jens Stoltenberg said reports of growing Russian military activity in Syria were a cause for concern, while France said it made finding a political solution to the crisis more complicated.Thus far in the war, Iran and its Lebanese ally Hezbollah have the main sources of military support for Assad, but momentum turned against him earlier this year.In the latest major battlefield setback, state television reported government troops had surrendered an air base in northwestern Syria to a rebel alliance after nearly two years under siege.The loss of the base meant the last government troops had now withdrawn altogether from central Idlib province, according to the Syrian Observatory for Human Rights, a UK-based group that monitors the conflict.Coalitions of rebels, who range from hardline Sunni Islamists like al-Qaida's Syrian affiliate the Nusra Front to more secular nationalists, have made gains in the northwest and southwest of the country. They often fight against both the government and Islamic State fighters who control much of the east of Syria as well as northern Iraq.Russia says the Syrian government must be incorporated into a shared global fight against Islamic State. The United States and Assad's regional foes see him as part of the problem.Russia's Foreign Ministry said Moscow would consider additional military measures needed for fighting terrorism in Syria if it deemed them necessary.A senior US official said on Saturday Washington had 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.Russia has in recent days set out the case for supporting Assad in the most forthright terms yet, likening the Western approach to Syria to failures in Iraq and Libya.Part of the diplomatic quarrel has centered around use of air space for flights, which Moscow says bring humanitarian aid but US officials say may be bringing military supplies.To avoid flying over Turkey, one of Assad's main enemies, Russia has sought to fly planes over Balkan states, but Washington has urged them to deny Moscow permission.On Tuesday Bulgaria refused a Russian request to use its airspace for flights due to doubts about the cargo on board. It said on Wednesday it would allow Russian supply flights to Syria to use its airspace only if Moscow agreed to checks of their cargo at a Bulgarian airport.Turkey has not officially confirmed a ban on Russian flights to Syria but says it considers any requests to fly over its air space to Syria on a case by case basis.On Wednesday the Syrian army withdrew completely from Idlib province after insurgents captured the Abu al-Duhur military airport there, said Rami Abdulrahman, head of the Syrian Observatory. Members of a local pro-government militia remained in just two Shi'ite villages in the province, he said.Rebel sources said the Nusra Front had played a leading role in the capture of the airport. Nusra is part of a coalition of Islamist groups called the Army of Conquest which has seized most of Idlib province this year.Syrian state television said in a news flash that the army garrison that had defended the military airport had evacuated.Another major base east of Aleppo, Kweiris, is currently besieged by ultra hardline Islamic State militants.Nusra Front made gains in northwestern Syria alongside other insurgent groups since May, seizing the city of Idlib, the town of Jisr al-Shughour and moving closer to coastal areas vital to government control of western Syria.