Oil-rich Deir Ezzor province borders Iraq and is a strategic prize for both the US-backed Syrian Democratic Forces (SDF) and Russian-supported government troops.
The SDF announced on Saturday it had begun clearing the Islamic State group from areas east of the Euphrates River, which cuts diagonally across the province.
By Sunday, fighters from the SDF's Deir Ezzor Military Council (DEMC) had seized much of the province's northeast, according to the Syrian Observatory for Human Rights.
"They seized control of a hilltop seven kilometres (four miles) from the eastern banks of the Euphrates, across the river from Deir Ezzor city," said Rami Abdel Rahman, who heads the Britain-based monitor.
He attributed the quick gains to the fact that "eastern parts of Deir Ezzor are desert and not densely populated".
The DEMC's advance is being backed by the US-led coalition battling IS in Iraq and Syria since 2014, while the regime's push on the city is being supported by its Russian allies.
The coalition, the SDF, Syria's government and Russia have agreed on a "de-confliction line" in northeastern Syria to prevent the two assaults from clashing.
Regime forces on Sunday were locked in fierce clashes with IS southwest of the city as they prepared a push into neighbourhoods still held by the jihadists, the Observatory said.
According to the War Media channel operated by regime ally Hezbollah, Syrian troops seized full control of the 450-kilometre (280-mile) road linking the capital to Deir Ezzor for the first time in four years.
"The Syrian army and its allies now control the entire international highway between Deir Ezzor and Damascus, through the cities of Al-Sukhna and Palmyra," it said, referring to other central Syrian cities recaptured from IS.
"The international highway had been unusable in the area between Al-Sukhna and Deir Ezzor for four years."
The Observatory said on Sunday that at least 21 civilians had been killed in Russian strikes targeting ferries crossing the Euphrates in the Al-Boulil district southeast of Deir Ezzor city.
Since 2014, IS has held about 60 percent of the city and much of the surrounding province.
A fighter from Deir al-Zor military council which fights under the Syrian Democratic Forces (SDF) holds the council's flag in the village of Abu Fas, Hasaka province, Syria
US-backed fighters made a sweeping advance against jihadists on Sunday in Syria's eastern Deir Ezzor province, capturing territory just kilometres from the provincial capital, a monitor said.
