Russian Foreign Minister, Sergei Lavrov, admitted on Thursday that flights delivering humanitarian aid to Syria also carry military equipment, after Western nations were alarmed by Russian cargo planes flying into the country.
Earlier this week, Bulgaria and Greece complied to a request from the US to prevent planes carrying supplies from Russia to Syria from using their airspace, due to suspicions they could be carrying weapons to Bashar al-Assad’s regime.
On Thursday, Ukrainian Prime Minister, Arseniy Yatsenyuk, said his country will also ban Syria-bound cargo planes from Russia from using its airspace. Iran, on the other hand, has allowed Russian planes to use its airspace.
Russia, an adamant ally of the Assad regime, has also sent military experts to Syria in order to train regime troops. However, Russia has denied accusations that it is planning to expand its military presence in Syria.
"Our partners - representatives of the Syrian armed forces - need some help and guidance," Deputy Foreign Minister, Mikhail Bogdanov, told the Russian Interfax news agency on Tuesday.
Russia recently sent two tank landing ships and an additional cargo aircraft to Syria, including the deployment of a small number of naval infantry forces, according to reports given by US officials, speaking to Reuters on the condition of anonymity.
They added that the intent of Russia's military advances into Syria was still unclear, but one possible reason could be to prepare an airfield near the port city of Latakia, which is currently a stronghold of Assad forces.
Presently, the Syrian coastal city of Tartus hosts Russia’s only repair and replenishment naval base in the Mediterranean. Russia has also denied that it plans to transform the port into a fully-fledged base.
However, the officials have not ruled out the possibility that Russia may want to use Syrian airspace for air combat operations.
Russia has not commented on whether or not the military experts are engaging in combat in Syria, but has insisted its help to the regime to fight ISIS militants is in line with international law.
"The threat coming from Islamic State is evident... The only force capable of resisting it is the Syrian armed forces," Kremlin spokesman Dmitry Peskov was quoted as saying to Reuters.
Over four years of fighting in Syria has left over 230,000 Syrians dead, according to UN estimates.
More than 6.7 million are displaced internally, while at least 5 million have fled the country to neighbouring countries of Turkey, Lebanon, Egypt and Jordan.
US, NATO slam Russia
NATO Chief, Jens Stoltenberg, stated that if Russia is deploying its army into Syria, then Moscow's involvement would not help to put an end to the conflict that killed more than 200,000 people and displaced more than 10 million people inside and outside the country.
Separately on Wednesday, US Secretary of State, John Kerry, reiterated his concerns to Russian Foreign Minister, Sergei Lavrov over the phone, warning that such advances could result in more violence.
A Russian Foreign Ministry spokeswoman, Maria Zakharova, insisted on Wednesday that this operation was nothing new, arguing that Moscow had been openly supplying weapons and sending military specialists to Syria for a long time.
"Russia has never made a secret of its military-technical cooperation with Syria,'' Zakharova said.
Assad’s government has also denied any military advances by Russian troops on Syrian soil.
Moscow has stated repeatedly that Assad’s government must be incorporated into the shared international battle against ISIS, which has taken over large parts of Syria and Iraq, even though the US and Assad's regional enemies view him and his regime as part of the problem.
"We would welcome constructive Russian contributions to the counter-ISIS 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.
US officials say that Russia’s increase in military assistance to Assad forces may be due to this year’s substantial territorial losses to ISIS militants, who have come to be seen as the bigger threat.