MOSCOW -- Iran has granted permission for Russian planes to fly over its territory en route to Syria, Russian news agencies said Wednesday, a bypass needed after Bulgaria refused overflights amid signs of a Russian military buildup in Syria that has concerned the U.S. and NATO.

The news agencies quoted Maxim Suslov, spokesman for the Russian Embassy in Tehran, as saying it has received Iranian permission for Syria-bound flights. After Bulgaria rejected Moscow's overflight request for Sept. 1-24, a path via Iran and Iraq appeared to be the only one left, as Russia apparently sought to avoid flying over Turkey, which in 2012 grounded a Syria-bound plane carrying radar parts from Moscow.

There was no immediate confirmation from Iran.

The controversy over the Russian flights comes amid signs of increased Russian military presence in Syria. Moscow, which has backed Syrian President Bashar Assad throughout the nation's 4 1/2-year civil war, said its military experts are in the country to train its military to use Russian weapons.

Russian Foreign Ministry spokeswoman Maria Zakharova accused the West of creating "strange hysteria" over Russian activities in Syria, saying that Moscow has been openly supplying weapons and sending military specialists there for a long time.

"Russia has never made a secret of its military-technical co-operation with Syria," she said, adding she could "confirm and repeat once again that Russian military specialists are in Syria to help them master the weapons being supplied."

President Vladimir Putin and other Russian officials have sought to cast arms supplies to Assad's regime as part of international efforts to combat the Islamic State group and other militant organizations in Syria.

Putin hasn't ruled out a bigger role. Asked Friday if Russia could deploy its troops to Syria to help fight IS, he said "we are looking at various options."

By playing with the idea of joining the U.S.-led coalition fighting IS, Putin may hope to reset ties with the West, which have been shattered by the Ukrainian crisis, and also protect Moscow's influence in Syria, where it has a navy base. But the U.S. and its allies have seen Assad as the cause of the Syrian crisis, and Washington has warned Moscow against beefing up its presence.

U.S. Secretary of State John Kerry spoke to Russian Foreign Minister Sergey Lavrov on Wednesday for a second time in five days. U.S. State Department spokesman John Kirby said Kerry "reiterated our concern about these reports of Russia military buildup," adding if they are true, it could lead "lead to greater violence and even more instability" in Syria.

Indicating a continuing rift, the Russian Foreign Ministry said that Lavrov on the call emphasized Syrian government troops' role in confronting extremist groups and the need to take consolidated action.

On Wednesday, NATO Secretary-General Jens Stoltenberg also said the alliance is concerned about reports about Russia's increased military presence in Syria. He didn't offer details.

A U.S. defence official, who spoke on condition of anonymity because he wasn't authorized to publicly discuss the issue, said the U.S. has seen the Russians fly a variety of military assets into the airfield south of the Syrian city of Latakia, including troops capable of protecting Russian forces there and modular housing units. He said it indicated that the Russians are preparing for some sort of air operations. The official said he was unaware of any evidence that Russian forces have conducted any offensive military operations in Syria.

Another U.S. official briefed on the latest intelligence declined to confirm or deny whether Russian troops have participated in military operations in Syria. However, he said, U.S. intelligence agencies have assessed that Russia's deployment of military personnel and weapons to Syria reflect growing concern about Assad's ability to weather opposition gains -- and it suggests that Moscow may be willing to intervene directly on Assad's behalf.

Russia's military involvement raises a number of concerns, the U.S. official said, especially because it does not appear to be co-ordinated with the other countries operating in the area. It is not clear what Russia intends to actually do, he said.

One Lebanese politician said on condition of anonymity because of the sensitivity of the issue that some Russian forces already have taken part in some small-scale operations in Syria, possibly paving way for broader military action against IS, including airstrikes, in the future. He provided no details, and other Lebanese politicians contested the claim, saying the Russians haven't joined the fray yet.

Another Lebanese politician familiar with the issue, who spoke on condition of anonymity because he wasn't in a position to publicly discuss the subject, said there are Russian experts and, possibly, pilots, in Syria, but no full-fledged fighting force yet.

"There are experts and there are also crews for advanced equipment," he said. "They have no fighting forces on the ground."

"Russia is a partner in the war," the politician added. "Russia from the beginning told several officials, including Lebanese, that defending Damascus is like defending Moscow. It will do what is needed."

Hisham Jaber, a retired Lebanese army general familiar with the Syrian military, also said Russian military experts have been in Syria for a long time.

"Every time Syria gets new weapons, Russian experts come to train them (Syrians) on these weapons," Jaber said. "Because of current situation in Syria, these experts need protection and special forces are in Syria to protect advanced weapons and to protect the Russian experts who train Syrians. There are plans to build a military air base in the coastal town of Jable."

Jaber said the Syrian coast is a "red line for the regime and the Russians," and it's threatened now after the fall of the northwestern town of Jisr al-Shughour earlier this year into the hands of al-Qaida fighters and their allies. Over the past weeks, militants have shelled the coastal city of Latakia. Jisr al-Shughour is only 50 kilometres (30 miles) away.

"The Russians will not allow the fall of the Syrian coast because of the naval base and the planned air base," Jaber said.

"Until this moment, there are no Russian forces fighting on the ground," he added. "There are experts everywhere, who sometimes give advice in operations rooms to Syrian forces."

Russian Deputy Foreign Minister Sergey Ryabkov criticized Washington for refusing to co-operate with the Syrian government in the fight against the IS.

"The basis for action of the U.S.-led anti-IS coalition is flawed, because it should at the very least involve co-operation with the countries on whose turf this battle is being fought," he said, according to Russian news agencies. "When our American colleagues manage to understand that there are global problems that can't be solved without Russia, we will be able to co-operate."