Russian Foreign Minister Sergey Lavrov has confirmed that the country's "humanitarian" flights to Syria carry military equipment as well as humanitarian aid - after the US and NATO warned Moscow over its involvement in the Syrian conflict.
"Russian planes are sending to Syria both military equipment in accordance with current contracts and humanitarian aid," Lavrov told reporters on Thursday.
Russia's Kommersant daily newspaper said earlier on Thursday that Moscow's advanced BTR-82A armoured personnel carriers were among arms supplied to Damascus.
Moscow has previously insisted in public that its flights to Syria were only for humanitarian purposes.
Al Jazeera's Peter Sharp, reporting from Moscow, said that nothing has changed and the Russians have been supplying the Syrian government for years now.
"Going back 60 years, Russia has been supplying Syria with arms, advisers, military equipment. Nothing much has really changed.
"Between 2009 and 2011 Russia was supplying 71 percent of Syria's military needs - everything from jets to military equipment to air defence systems.
"The Russian foreign minister says this continues to take place but he did make a distinction: There has been additional air traffic coming into Latakia's airbase and he says military equipment and humanitarian aid are being delivered," Sharp said.
"As far as boosting up boots on the ground, he said Russian military specialists are working on training Syrians on using Russian weapons and no additional steps have been taken."
The Kremlin declined to comment on Thursday on whether Russian troops were fighting in Syria, after sources in Lebanon told the Reuters news agency that Russian forces had begun participating in military operations there.
"The threat coming from Islamic State [ISIL] is evident... The only force capable of resisting it is the Syrian armed forces," said Kremlin spokesman Dmitry Peskov, reiterating Russia's position that its longtime ally, Syrian President Bashar al-Assad, should be part of international efforts to combat ISIL.
Washington has put pressure on Greece and Bulgaria in recent days to deny Russia's requests to use their airspace for its Syria flights.
During a press conference with the Slovakian prime minister, Ukrainian Prime Minister Arseny Yatsenyuk announced the country will close its airspace to Russian planes flying to Syria.
On Wednesday, US Secretary of State John Kerry expressed his concern over reports of Russian military activities in Syria, warning that it could fan more violence, a state department spokesman said.
Al Jazeera's Parry Culhane, reporting from Washington, said officials in the White House have not yet commented.
"What officials here are saying is that it is simply wrong to help the government of President Bashar al-Assad because of what he has done to his people.
"There is also a big concern that there could be some sort of accident where the US-led coalition could somehow run into a fight with the Russians," Culhane added.
Aerial imagery indicated that Russia is focusing on Bassel al-Assad International Airport, south of Latakia on Syria's Mediterranean coast, and on the Russian naval facility in Tartus, the AFP news agency reported.
Jens Stoltenberg, the NATO secretary-general, expressed a similar reaction to Kerry, saying the move "will not contribute to solving the conflict".
"I think it is important to now support all efforts to find a political solution to the conflict in Syria. We support very much the efforts by the UN."
One US official, speaking on condition of anonymity, told the AFP news agency that two tank-landing ships have recently arrived at Tartus and about a dozen Russian armoured personnel carriers are now at the Bassel al-Assad airport, named after Bashar al-Assad's older brother.
The official said dozens of Russian naval infantry had arrived in Syria, but their role was likely to protect incoming military hardware rather than a boots-on-the-ground deployment.
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In addition, another giant Antonov-124 Condor military transporter had flown into the airport, bringing the total number of transport flights to at least four in recent days.
The developments further complicate the deadly crisis in Syria, where the conflict has claimed nearly 250,000 lives since 2011 and triggered a massive outflow of refugees - many of whom are fleeing to Europe.
In an interview with Al Jazeera, Mamoun Abu Nowar, a retired Jordanian air force general, said on Thursday that Russia is sending fighter jets to support the Syrian military.
"It is clearly obvious that Assad's forces have faced setbacks. So this Russian movement sends a great signal that Assad must not go," he said, adding that an estimated 1,000 "prefabricated" houses have been built in Syria to accommodate Russian troops.
Source: Al Jazeera and agencies