NATO General Philip Breedlove expressed concern Monday about the strength of Russia's military build-up in northwestern Syria and the apparent creation of a defensive "bubble" in the Mediterranean.

The supreme allied commander in Europe for the 28-member military alliance said Russia had sent advanced weaponry beyond what is needed to fight the Islamic State group -- meaning the hardware is to protect Syrian President Bashar al-Assad.

"We see some very sophisticated air defenses going into these airfields. We see some very sophisticated air-to-air aircraft going into these airfields," Breedlove told an audience at the German Marshall Fund in Washington.

"I have not seen (the Islamic State) flying any airplanes that require sophisticated air-to-air capabilities."

The Pentagon says Russia has sent at least 500 troops, along with fighter jets, artillery units, tanks and other military hardware to an airbase in the Latakia region on Syria's Mediterranean coast.

Breedlove suggested the weaponry included SA15 and SA22 surface-to-air missile defense systems, used to take down enemy planes.

"I have not seen ISIL flying any airplanes that require SA15s or SA22s," he said, using an alternative acronym for the IS group.

Russian President Vladimir Putin and his US counterpart Barack Obama sparred over the crisis in Syria in dueling UN speeches on Monday, each accusing the other of fueling the carnage.

The two leaders were due to meet on the sidelines of the UN General Assembly later.

Breedlove said Russia was trying to protect Assad's regime "against those that are putting pressure on" it.

Using military jargon, he warned of Russia forming an "A2AD" (Anti-Access Area-Denial) exclusion zone.

"It's one of the things we are beginning to watch (them) develop in the northeast Mediterranean as we see these very capable air defense capabilities beginning to show up in Syria," Breedlove said.

"We are a little worried about another A2AD bubble being created in the eastern Mediterranean."

He noted Russia already had created such a zone in the Black Sea, thanks to missile batteries sent to Crimea after its annexation by Russian forces.

They are also using the Russian enclave of Kaliningrad to create a bubble over the Baltic, he said.

Breedlove's comments come as the West frets over Russia's intentions in Syria and in eastern Ukraine, where it is supporting pro-Moscow rebels in an ongoing conflict.

"Russia very much wants to be seen as an equal on the world stage, a great power on the world stage," he said.

Moscow "wants to maintain warm water ports and airfield capabilities in the eastern Mediterranean and they saw that possibly being challenged on the ground by those opposing the Assad regime."