BEIRUT (Reuters) - Syria’s army and its allies neared al-Mayadin on Friday, sources said, pushing an advance on an eastern city seen as Islamic State’s main remaining base in the country.

The government forces had reached within 10 km (7 miles) of the city and taken positions parallel with a key road, a military media unit run by the Lebanese Hezbollah group said .

The Syrian Observatory for Human Rights, a war monitor, said the army supported by Russian forces had reached the western edge of al-Mayadin, entering the ruins of a castle and an area of grain silos.

Islamic State has lost swathes of territory in recent months to the Syrian government, backed by Russian air power and Iran-allied militias, in eastern Deir al-Zor province that borders Iraq.

A Syrian military source told Reuters fighting was ongoing around Deir al-Zor towards al-Mayadin on Friday.

“The Syrian army is continuing to strike the terrorist Daesh (Islamic State) group fiercely in Deir al-Zor and its surroundings ... and will continue its operations until it eliminates (Daesh) and drive what’s left of them out of this province,” he said.

Last month the Syrian army’s advance to Deir al-Zor city lifted a three-year-long siege imposed by Islamic State on a government-held enclave there.

The army has since pushed its advance further eastwards towards al-Mayadin, which lies 45 km south of Deir al-Zor, along the Euphrates valley.

U.S.-backed Syrian militias are also waging a separate campaign against the jihadist group’s last major strongholds in Deir al-Zor province

In June, two U.S. intelligence officials told Reuters that they believed Islamic State had moved most of its diminished command structure and propaganda team to al-Mayadin, southeast of its former capital of Raqqa.

The Observatory said on Friday there were clashes in several areas in eastern Deir al-Zor province. The Syrian military shelled al-Mayadin overnight and Syrian and Russian warplanes have conducted hundreds of strikes, it said.

Hezbollah, part of the military alliance supporting President Bashar al-Assad, said this week a senior commander had been killed in an Islamic State attack in Syria’s central desert.