Jihadists advanced on a regime airbase in eastern Syria on Thursday after the fall of one in the north, as staunch ally Russia defended its military aid to embattled President Bashar al-Assad.
Moscow dismissed criticism of its intervention by Western states, which are carrying out air strikes on jihadists in both Syria and neighbouring Iraq but also seek Assad's ouster.
"We helped, are continuing to help and will help the Syrian government when it comes to supplying the Syrian army with everything it needs," said Russian Foreign Minister Sergei Lavrov.
Islamic State group jihadists edged closer to a strategic airbase outside the eastern city of Deir Ezzor in heavy clashes that left 54 combatants dead, said the Syrian Observatory for Human Rights.
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 (less than a mile) 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, said Abdel Rahman.
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.
- Three bases left -
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 northwestern 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.
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At least 56 soldiers were killed, some execution-style, as rebels captured Idlib's Abu Duhur military airport under the cover of an intense sandstorm following a two-year battle, the Observatory said.
Abdel Rahman, whose Britain-based group relies on a network of sources on the ground in Syria, said at least 40 soldiers were taken prisoner and dozens more were missing.
Rebels tweeted images of helicopters and planes abandoned on the tarmac.
Abdel Rahman said the entire province of Idlib was now under the control of Al-Nusra Front and other rebel groups.
Assad's regime appeared to admit the loss, with state television saying troops had left the base.
It has been at war with different rebel groups since March 2011, in a conflict that has killed at least 240,000 people and forced millions to flee abroad.
Russia denied it was ramping up its military presence in Syria, 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," Lavrov told reporters in Moscow.
"We have never made our military presence (in Syria) a secret," he said. "Russia is not taking any additional steps."
US officials said this week that Russia was solidifying its foothold in Syria, sending ships, armoured personnel carriers and naval infantry.
French Foreign Minister Laurent Fabius said he was "concerned" by US reports of a Russian military buildup and reiterated that there could only be a political solution to Syria's conflict.
Lavrov for his part rubbished suggestions that Russia's greater involvement in Syria would throw a wrench in the plans of the Western coalition's fight against IS.
"This logic is uncomprehensible to me," Russia's top diplomat said. "The Syrian army is the most effective force that can stand up to the terror threat on the ground."
On the diplomatic front, diplomats at the United Nations said Moscow has lifted its objections to a UN investigation into chemical attacks in Syria, clearing the way for the probe to begin.
The UN Security Council unanimously adopted a resolution August 7 approving a joint investigation by the UN and the Organisation for the Prohibition of Chemical Weapons.