Regime forces on Friday broke into the eastern town of Mayadeen, one of ISIL's last bastions in Syria, backed by Russian air raids taking a deadly toll on civilians.
Mayadeen in the oil-rich eastern province of Deir Ezzor is seen as the jihadist group's "security and military capital" in Syria, and its loss would deal "a severe blow" to the jihadists, according to a Syrian military source.
Over the course of months of successive defeats, Mayadeen and nearby Albu Kamal on the Iraqi border have taken in ISIL fighters fleeing the battle to the north for Raqqa city in the face of an offensive launched by US-backed Kurdish and Arab forces.
"With support from Russian aviation, regime forces entered Mayadeen and took control of several buildings in the west of the town," Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, told AFP.
Mayadeen, which the jihadists have controlled since 2014, sits on the western bank of the Euphrates River, between provincial capital Deir Ezzor, where the jihadists still hold several districts, and the border with Iraq.
ISIL remains in control of half of Deir Ezzor province, despite advances by President Bashar Al Assad's forces and a separate offensive against the jihadists by the Kurdish-Arab alliance.
The Observatory said the target of the regime advance was to recapture the Al Omar oilfield held by ISIL to the northeast of Mayadeen that was destroyed in US-led coalition air strikes in 2015.
The jihadists had been drawing oil sale revenues from the field of between $1.7 million and $5.1 million a month, according to the coalition.
Civilians killed in air strikes
On another front, regime forces said on Friday they had ended their military operations in the east of central province of Homs, after "eliminating the last groups" of ISIL fighters from an area of 1,800 square kilometres, the official Sana news agency reported.
The advances against ISIL in Deir Ezzor have cost a heavy civilian death toll from Russian and coalition air raids.
The Observatory said Russian air strikes on Thursday night killed 14 people, including three children, fleeing across the Euphrates on rafts near Mayadeen.
Moscow has been carrying out relentless air strikes in support of its ally Damascus targeting both ISIL in Deir Ezzor province and rival jihadists led by Al Qaeda's former Syria affiliate in Idlib province in the northwest.
ISIL has seen its self-declared "caliphate" straddling Syria and Iraq shrink steadily over the past two years and has lost all but a few of its main hubs in both Arab states.
On Wednesday, another Russian air strike killed 38 civilians trying to flee the fighting in Deir Ezzor province, according to the Observatory.
The Observatory relies on a network of sources inside Syria, and says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.
It has reported hundreds of civilians killed in anti-ISIL operations in Deir Ezzor and Raqqa. On Tuesday, it said a US-led coalition strike in Raqqa killed at least 18 civilians.
Russia has not acknowledged any civilian deaths from its strikes since it intervened in Syria in 2015, and dismisses the Observatory's reporting as biased.
On Thursday, the Red Cross said Syria was experiencing its worst levels of violence since the battle for the country's second city Aleppo late last year.
"For the past two weeks, we have seen an increasingly worrying spike in military operations that correlates with high levels of civilian casualties," said Marianne Gasser, the head of the International Committee of the Red Cross delegation in Syria.