NATO and the US have warned Russia against building up its military presence in Syria after reports Russian ground troops have been involved in fighting there.
NATO chief Jens Stoltenberg said any increase in Russian support for the embattled Syrian government would fuel, not help solve, the four-year-long war.
But Russia denied it was ramping up its military presence, saying it was supplying its ally with humanitarian aid and military equipment under existing contracts.
“Russian planes are sending to Syria both military equipment — in accordance with current contracts — and humanitarian aid,” Russian foreign minister Sergei Lavrov said.
“We have never made our military presence [in Syria] a secret. Russia is not taking any additional steps.”
Mr Lavrov rubbished suggestions Russia’s greater involvement in Syria would throw a spanner in the plans of the Western coalition’s fight against Islamic State (IS) militants.
“This logic is incomprehensible to me,” he said.
“The Syrian army is the most effective force that can stand up to the terror threat on the ground.”
NATO and US warnings follow reports Moscow has deployed additional aircraft and two tank landing ships in the past couple of days.
US secretary of state John Kerry called his Russian counterpart, warning if the reports were true it could lead to greater violence.
The German and French foreign ministers also registered their concerns.
Jihadists close in on another airbase
On the ground in Syria, jihadists advanced on a regime airbase in the east after the fall of one in the north.
The Syrian Observatory for Human Rights said 54 combatants died in heavy clashes as IS fighters closed in on the airbase outside the city of Deir Ezzor.
The Sunni Muslim group, which has captured large chunks of Syria and Iraq, seized control of an army post near the base on Wednesday night.
Observatory director Rami Abdel Rahman said 36 IS fighters and 18 regime soldiers were killed.
IS fought its way to barely one kilometre from the airport with the seizure of the army post used by a rocket battalion.
Two suicide bombers were used by the jihadists in the assault, one of them a child, driving cars laden with explosives, Mr Abdel Rahman said.
IS already controls most of oil-rich Deir Ezzor province, including about half of its capital, and has fought for more than a year to capture the airport and the rest of the city.
The Deir Ezzor assault came as rival jihadists from Al Qaeda’s Syrian affiliate and their allies on Wednesday seized the last regime-held military base in the north-western province of Idlib.
According to the Observatory, the regime is now left with just three airbases in the east and north — Deir Ezzor, and Neirab and Kweyris in Aleppo province.