Fighting between Islamic State and the Syrian army has killed dozens since Saturday in Deir al-Zor, where the militant group has launched an assault to capture a government enclave in the city, a monitoring group reported.

At least 82 people have been killed in the fighting, which is the heaviest in the city for a year, the Syrian Observatory for Human Rights said on Monday.

The dead comprised 28 from the army and allied militia, at least 40 Islamic State fighters, and 14 civilians, the British-based monitoring group said.

Syrian state news agency SANA said the army had killed dozens of Islamic State fighters in attacks on the group's positions around Deir al-Zor.

Islamic State has held most of Deir al-Zor and the surrounding area since 2015, but the government has retained control of the airport and neighboring districts in the city, located in eastern Syria on the Euphrates river.

A U.S.-backed coalition of Kurdish and Arab militias and rival Turkish-backed Syrian rebel groups have pushed Islamic State from much of its territory in northern Syria, but it remains embedded in the country's eastern desert and Euphrates basin.

Last month it recaptured the city of Palmyra, 185km (115 miles) southwest of Deir al-Zor, from the government in an unexpected advance that demonstrated its continuing military threat.