Russian soldiers operating on an armored personnel carrier
Russian military experts are in Syria overseeing arms deliveries from Moscow. Pictured: Russian navy servicemen left an armored personnel carrier as they took part in the Caspian Derby 2015 international naval contest during the International Army Games in Kaspiysk, Russia, on Aug. 5, 2015. Sergei Karpukhin/Reuters

Iran gave permission Wednesday for Russia to use its airspace for the delivery of humanitarian aid to Syria, according to a report from the Russian website Sputnik. Tehran's decision to open up its skies to Moscow's planes came just a day after the U.S. asked Bulgaria and Greece to shut down their airspace to Russian transport planes bound for Syria. The U.S. fears that Moscow is supplying Syria with more than just humanitarian aid -- the Russian government's stated purpose of the flights, according to the New York Times.

"Several requests have been sent [to allow Russian aid flights to Syria via Iran's airspace], and the Iranian side approved all of them," Maksim Suslov, Russia's attache in Tehran, said.

Russia also confirmed Wednesday that it has soldiers based in Syria. The confirmation is the first time that Russia has admitted that it has military personnel in Syria and came after weeks of international speculation that Russia was growing its forces in the Middle Eastern country, Reuters reported.

The statement claimed that the soldiers are in Syria to coordinate the delivery of weapons and not there in an official combat capacity.

Russia is making preparations to deploy military personnel and aircraft to an airfield south of Latakia, where it will launch airstrikes against the Islamic State, also known as ISIS, an American official told the New York Times, adding that Russia is “clearly establishing some sort of forward operating base" in Syria.

While both the U.S. and Russia agree that the Islamic State is a threat, they do not see eye to eye on how to deal with the Bashar Assad regime. "Any military support to the Assad regime for any purpose, whether it's in the form of military personnel, aircraft supplies, weapons or funding, is both destabilizing and counterproductive,” White House spokesmen John Earnest told reporters last week.

Last week, ISIS admitted attacking a Russian military base in the Dagestan region of the North Caucasus. It was the first attack by the terrorist group on Russian soil and came amid speculation that it had begun heavily recruiting from the primarily Muslim regions in southern Russia.

In the wake of the attack, Chechen leader Ramzan Kadyrov, an ally of Russian President Vladimir Putin, vowed to crush the terror group. "I want to remind everyone who is planning something against our country that Russia has worthy sons ready to fulfill any order, wring the neck of any enemy in his own lair, wherever he may be," Kadyrov said. "And we find ourselves with happiness ridding the world of these scum."