An Islamic State group attack Saturday in the eastern Syrian city of Deir Ezzor killed at least 85 civilians and 50 regime forces, a monitor said, with local media denouncing a "massacre".
Syria's state news agency SANA, quoting residents, said "around 300 civilians" were killed in the onslaught.
If confirmed it would be one of the highest tolls for a single day in Syria's nearly five-year war.
The bloodshed in Deir Ezzor came as regime forces battled IS in the northern province of Aleppo, killing at least 16 jihadists, and as air strikes hit the IS stronghold of Raqa.
The Syrian Observatory for Human Rights said IS had advanced into the northern tip of Deir Ezzor city and captured the northern suburb of Al-Baghaliyeh.
Initially it reported that 35 Syrian soldiers and allied militiamen were killed in the multi-front attack, which including a suicide bombing.
But as the day unfolded the death toll rose, with the Britain-based monitor saying that civilians were among those killed in Deir Ezzor.
It said most of the victims were killed execution-style in Al-Baghaliyeh.
Quoting "local sources", SANA denounced a "massacre".
"The Daesh (IS) terrorists carried out a massacre in Al-Baghaliyeh, claiming the lives of around 300 civilians, most of them women, children and elderly people," the agency said.
Rebel fighters of the al-Sham Brigade take part in a training session in the northeastern city of Deir Ezzor, on March 25, 2014.
It quoted Syrian Prime Minister Wael al-Halaqi as saying that the "legal and moral responsibility for this barbaric and cowardly massacre... lies on the shoulders of all the states that support terrorism and that fund and arm takfiri (Sunni extremist)" groups.
According to the Observatory, the advance puts IS in control of around 60 percent of Deir Ezzor city, capital of the province of the same name in an oil-rich region bordering Iraq.
IS said its fighters carried out several suicide bombings against regime forces in Deir Ezzor and seized control of Al-Baghaliyeh and other areas.
The Observatory said Russian warplanes were carrying out heavy air strikes in support of regime forces as they sought to repel the jihadists.
Regime troops were locked in fierce clashes with IS in Aleppo province, with at least 16 jihadists killed after a failed attack on a government position near the town of Al-Bab, the monitor said.
State television also reported that regime forces had repelled an assault in the town.
The Observatory said heavy fighting was ongoing Saturday in the area, with Russian warplanes carrying out strikes in the region between the regime-held Kweyris air base and Al-Bab.
Syrian regime forces fire towards Islamic State group jihadists south of the town of Al-Bab, in the northern province of Aleppo on January 14, 2016.
The regime has advanced towards the town, an IS bastion, in recent days, and is now within 10 kilometres (six miles) of it, said the Observatory.
That is the closest regime forces have come to Al-Bab since 2012.
Located some 30 kilometres south of the Turkish border, Al-Bab fell into rebel hands in July 2012, and IS jihadists captured it in late 2013.
The fighting in Al-Bab is just one of up to seven battlefronts on which regime forces are seeking to advance in Aleppo province, capitalising on a Russian air campaign that began on September 30.
The battles are intended in part to cut rebel supply lines into Aleppo city, the provincial capital and Syria's second city.
Aleppo itself is divided and regime forces are now hoping to effectively encircle the opposition-held east.
In addition to cutting rebel access to eastern Aleppo city, the regime is hoping to sever areas controlled by IS in the province from its territory in neighbouring Raqa, Abdel Rahman said.
Syrian men inspect a damaged vehicle following a reported air strike by government forces in the northern city of Aleppo, on January 16, 2016.
Raqa, the self-declared capital of IS, has come under frequent air strikes by the US-led coalition, the Syrian air force and Russian warplanes.
On Saturday at least 16 people, including civilians, were killed in air strikes and 30 others were wounded, said Abdel Rahman.
He said eight strikes hit the city and its surroundings but did not specify who carried them out.
British Foreign Secretary Philip Hammond, in comments reported Saturday, said some 600 Britons have been stopped from going to Syria to join IS and other jihadist groups.
Hammond said these interceptions as well as air strikes were placing extra strain on IS in its Raqa headquarters.
"There is evidence (IS) is finding it difficult to recruit to the brigades in Raqa because of the high attrition rate of foreign fighters," he said, according to The Guardian and The Daily Telegraph newspapers.
Syria's war has killed more than 260,000 people and forced millions to flee their homes.