Syrian army artillery guns are stationed near the village of Huraybishah, within the administrative borders of Syria's eastern Deir Ezzor province, on September 3, 2017

Syrian government soldiers and allied fighters have advanced to the edge of a government enclave besieged by the Islamic State group in the country's east, a monitoring group said Sunday.

Syria's army, backed by Russian military support, has been advancing towards the city of Deir Ezzor on several fronts for weeks.

Late Sunday, the Syrian Observatory for Human Rights said government soldiers had arrived at the Brigade 137 base on the city's edge and were battling to break the IS siege of the facility.

"Soldiers arriving from outside and the soldiers besieged inside are fighting IS, but they (jihadists) haven't withdrawn yet," said Observatory director Rami Abdel Rahman.

"If the regime soldiers succeed, they will have broken the siege on the base and the neighbouring Deir Ezzor district of Al-Jura," he said.

That would still leave other parts of the city and its military airport besieged, he added.

There was no official confirmation from Syrian state media or military sources of the advance.

The group had earlier reported that the Syrian army and its allies were within 10 kilometres (six miles) of the base and had seized the Al-Kharata oilfield in the resource-rich Deir Ezzor province.

IS jihadists have held much of the province and parts of its capital, the city of Deir Ezzor, for years.

Government forces retain control of segments of the city but they have been under siege by IS fighters since 2015.

The city's residents have survived in part through airdropped humanitarian assistance during the siege.

Syria's army is advancing towards the enclave on several fronts, including from the provinces of Raqa to the west and from Homs to the south.

Meanwhile, a convoy consisting of IS fighters and civilians evacuated from the Lebanon-Syria border that was headed towards Deir Ezzor remains stranded short of its destination, the US-led coalition fighting the jihadists said.

Members of the Syrian government forces stand on the entrance to Deir Ezzor city on September 3, 2017

The convoy of 17 buses started out on Monday for the IS-held town of Albu Kamal under a deal negotiated by Syria's government and its Lebanese ally Hezbollah, a militant group.

But the US-led coalition bombed the convoy's route and targeted IS fighters attempting to reach the buses, insisting that it would not allow militants 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 others heading west towards Palmyra in central Homs, an area held by Syria's government.

It said it had offered a plan to spare the women and children in the convoy "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," the coalition said, using another acronym for IS.

The statement said coalition strikes had not targeted the convoy, but had killed around 85 IS fighters seeking to facilitate the movement of the buses.

Hezbollah has defended the deal, and accused Washington of facilitating similar IS withdrawals elsewhere.

But Iraq's government criticised the agreement as "unacceptable".