Syria's army and its allies have advanced towards ISIL-held Deir Az Zor city after capturing the al-Kharata oil field, a war monitor said on Sunday.

Syrian President Bashar al-Assad has focused this year on striking eastwards in a multipronged assault against ISIL, or the Islamic State of Iraq and the Levant (also known as ISIS) group, to reclaim Deir Az Zor from the armed group.

ISIL controls most of Deir Az Zor province - apart from a Syrian government-held enclave in Deir Az Zor city and a nearby military airbase.

Sunday's advance brings the army and its allies to about 10km from the city, said the Britain-based Syrian Observatory for Human Rights (SOHR).

The governor of Deir Az Zor province said the Syrian army would reach the city within 48 hours.

"The heroes of the army will arrive at Deir Az Zor in 24-48 hours at the most," Mohammed Ibrahim Samra told Reuters news agency, adding that the army was now 18-20km from the edge of the city.

On Saturday, a military media unit run by Assad's ally Hezbollah reported the army had captured Jebel al-Bashri to the west of Deir Az Zor, meaning forces were less than 30km from the city.

Pro-government fighters were also battling to eradicate a large ISIL enclave they left to their rear in central Syria as they move towards Deir Az Zor.

Meanwhile, ISIL fighters and their families evacuated from the Lebanon-Syria border and headed towards Deir Az Zor remain stranded short of their destination, the US-led anti-ISIL coalition said.

A convoy of 17 buses started out on Monday headed for the ISIL-held town of Albu Kamal under a deal negotiated by Syria's government and Lebanese Hezbollah.

However, the coalition bombed the convoy's route and targeted ISIL fighters attempting to reach the buses, insisting it would not allow fighters to reach Albu Kamal on the border between Syria and Iraq.

On Sunday, the coalition said the convoy had split into two groups, with some buses remaining in the open desert northwest of Albu Kamal, and the rest headed west towards Palmyra in central Homs, an area held by Syria's government.

The anti-ISIL coalition said it offered a plan to save women and children in the convoy from "further suffering", but gave no details.

"We will continue to monitor the convoy, but not allow it to link up with ISIS in the Euphrates River Valley," it said, using a different acronym for ISIL.