Jihadists have advanced on a regime air base in eastern Syria 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 air base outside the eastern city of Deir Ezzor in heavy clashes that left 54 combatants dead, said the Syrian Observatory for Human Rights.
The extremist Muslim group, which has captured large chunks of Syria and Iraq, seized control of an army post near the base on Wednesday night (local time).
Observatory director Rami Abdel Rahman said 36 IS fighters and 18 regime soldiers were killed.
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.