
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 the United 
States fears is a deepening Russian military role in Syria's civil war, 
though one of the Lebanese sources said the number of Russians involved 
so far was small.
U.S. 
officials said Russia sent two tank landing ships and additional cargo 
aircraft to Syria in the past day or so and deployed a small number of 
naval infantry forces.
The 
U.S. officials, who also spoke on condition of anonymity, said the 
intent of Russia's military moves in Syria was unclear. One suggested 
the focus may be on preparing an airfield near the port city of Latakia,
 a stronghold of Syrian President Bashar al-Assad.
U.S. officials have not ruled out the possibility that Russia may want to use the airfield for air combat missions.
U.S.
 Secretary of State John Kerry spoke to his Russian counterpart for the 
second time in four days to express concern over reports of Russian 
military activities in Syria, warning that it could fan more violence.
The White House said it was closely monitoring the situation.
Russia
 says the Syrian government must be incorporated into a shared global 
fight against Islamic State, the Islamist group that has taken over 
large parts of Syria and Iraq. The United States and Assad's regional 
foes see him as part of the problem.
"We
 would welcome constructive Russian contributions to the counter-ISIL 
effort, but we've been clear that it would be unconscionable for any 
party, including the Russians, to provide any support to the Assad 
regime," White House spokesman Eric Schultz said, using an acronym for 
Islamic State.
SYRIAN TROOPS PULLING BACK
Assad's
 forces have faced big 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 last 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 in Syria, its long-time ally in the Middle East.
But
 Russia has declined to comment on the scale and scope of its military 
presence. Damascus denied Russians were involved in combat, but a Syrian
 official said the presence of experts had increased in the past year.
Reflecting
 Western concern, Germany's foreign minister warned Russia against 
increased military intervention, saying the Iran nuclear deal and new 
U.N. 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.
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 advisors," one of the sources said. "The 
Russians have decided to join the war against terrorism."
RUSSIAN NAVAL BASE
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.
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."
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".
Russia has set 
out the case for supporting Assad in the most forthright terms yet in 
the past few days, likening the Western approach to Syria to failures in
 Iraq and Libya.
Part of the 
diplomatic quarrel has centred around use of air space for flights, 
which Moscow says bring humanitarian aid but U.S. officials say may be 
bringing military supplies.
U.S.
 officials, speaking on condition of anonymity, told Reuters on 
Wednesday that multiple Russian flights have passed over the airspace of
 Iran and Iraq to reach Syria.
RUSSIAN OVERFLIGHTS
The State Department said Russian use of Iranian airspace would not be surprising, given Tehran's past support for Assad.
Spokesman
 John Kirby said the United States had advised "partners and our friends
 to ask the Russians tough questions about" overflight requests. He did 
not elaborate, saying only: "I'm not gonna detail diplomatic 
conversations."
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 citing 
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.
Thus far in 
the war, Iran and its Lebanese ally Hezbollah have been Assad's main 
sources of military support. The momentum turned against Assad earlier 
this year.
In the latest 
setback, state television reported government troops had surrendered an 
air base in northwestern Syria to a rebel alliance after nearly two 
years under siege.
This meant
 the last government troops had withdrawn from central Idlib province, 
according to the Syrian Observatory for Human Rights, a UK-based group 
that monitors the conflict.   (Additional reporting by Suleiman 
al-Khalidi in Amman; Mark Hosenball and Lesley Wroughton in Washington; 
Writing by Tom Perry, Sylvia Westall, Peter Graff and Phil Stewart; 
Editing by Giles Elgood, David Storey, Grant McCool)