Islamic State jihadists advanced for the first time on Sunday into a military airport that is the last stronghold of the Syrian army in Raqa province, a monitoring group said.
The Syrian Observatory for Human Rights said fighters from the group had launched a fourth attack in six days on Tabqa military airport overnight and entered the facility Sunday morning.
"IS launched a new attack on the airport, which is the fourth since Tuesday," said Rami Abdel Rahman, director of the Britain-based Observatory.
"They were able to enter it for the first time, but they don't yet control the facility," he added.
Abdul Rahman said the jihadists had also seized several checkpoints, hanging up at one the head of a soldier who had been killed in the fighting and decapitated.
There was no immediate death toll in the latest fighting that began overnight, but at least 100 Islamic State fighters and 25 regime troops had been killed before the new clashes.
Regime forces have repelled three previous attacks on Tabqa air base.
Regime warplanes have backed forces on the ground and carried out six new raids on Sunday on different targets.
The battle for Tabqa began earlier this month, with IS launching its first major assault against the airport on Tuesday.
It is the last army stronghold in the Raqa, after jihadists captured Brigade 93 and Division 17 in the northern province, killing dozens of soldiers, many of whom were beheaded.
The army has airlifted reinforcements to the base and stepped up air strikes against IS positions across Raqa, using both precision rockets and barrel bombs.
Raqa has become the stronghold of the Islamic State, which controls the provincial capital and has declared an Islamic "caliphate" in territory it holds in Syria and Iraq.
The group initially fought alongside Syrian opposition groups, but its abuses sparked a backlash from rebels who pushed it out of parts of northern Syria.
In recent weeks, though, the group has advanced back into areas it withdrew from, including northern Aleppo province.
Elsewhere, however, the Observatory said IS fighters were withdrawing from parts of central Homs province.
The monitoring group said there was no official reason given for the withdrawal, but that the fighters appeared to be moving to areas under tighter IS control, including Deir Ezzor province in the east.
The Observatory said several areas in Homs that had been under IS control were taken over in the wake of the withdrawal by rival jihadist group Al-Nusra Front, which is Al-Qaeda's Syrian affiliate.