The Syrian army and its allies are fighting to secure and expand a precarious corridor to their comrades in Deir al-Zor a day they smashed through Islamic State (IS) militants lines to break the Islamist militant siege.

The army reached Deir al-Zor on Tuesday in a sudden, days-long thrust that followed months of steady advances east across the desert, breaking a siege that had lasted three years.

However, IS counter-attacks lasted through Tuesday night, the British-based Syrian Observatory for Human Rights reported, as the Islamist militants tried to repel the army.

It points to a tough battle ahead as the army aims to move from breaking the siege to driving IS from its half of the city, the sort of street-by-street warfare in which the Islamist militants excel.

"The next step is to liberate the city," a non-Syrian commander in the military alliance backing President Bashar al-Assad said.

Assad and his allies -- Russia, Iran and Shia militias including Hezbollah -- will follow the relief of Deir al-Zor with an offensive along the Euphrates valley, the commander said.

The Euphrates valley cuts a lush, populous swathe of green about 260 km (160 miles) long and 10 km (6-7 miles) broad through the Syrian desert from Raqqa to the Iraqi border at al-Bukamal.

The area has been an IS stronghold in Syria but came under attack this year when an alliance of Kurdish and Arab militias backed by a U.S.-led coalition besieged and assaulted Raqqa.

Rapidly losing territory in both Syria and Iraq, IS is falling back on the Euphrates towns downstream of Deir al-Zor, including al-Mayadin and al-Bukamal, where many expect it to make a last stand.

However, the Islamist militant group specialises in urban combat, using car bombs, mines, tunnels and drones, and has held out against full-scale attack for months in some towns and cities.

FIGHTING

Parallel with their thrust towards Deir al-Zor, the Syrian army and its allies have been fighting IS in its last pocket of ground in central Syria, near the town of al-Salamiya on the Homs-Aleppo highway.

On Wednesday, army advances gained control of four villages in that area, further tightening the pocket, a military media unit run by Assad's ally Hezbollah reported.

In Raqqa, the Syrian Democratic Forces alliance, backed by the U.S.-led coalition, has taken about 65 percent of the Islamist militants' former de facto capital in Syria, it has said.

Deir al-Zor lies along the southwest bank of the Euphrates. The government enclave includes the northern half of the city and the Brigade 137 military base to the west.

The government also holds an air base and nearby streets, separated from the rest of the enclave by hundreds of metres of IS-held ground and still cut off from the advancing army.

Instead of breaking the siege along the main road from Palmyra, stretches of which remain in IS hands, the army reached the Brigade 137 along a narrow salient from the northwest.

"Work is progressing to secure the route and widen the flanks so as not to be cut or targeted by Daesh," the commander said, using the Arabic acronym for IS.

The route from the west into Brigade 137 is only about 500 metres (yards) wide, the commander said.

IS counter-attacks in that area managed to cut the corridor into the enclave for several hours on Tuesday night using six car bombs, the Observatory reported.

The army will also push towards the still besieged airbase, southwards from the Brigade 137 camp and eastwards along the main highway, the commander said.