By Laila Bassam and Angus McDowall

BEIRUT (Reuters) – 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 lines to break the jihadist 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, Islamic State counter-attacks lasted through Tuesday night, the British-based Syrian Observatory for Human Rights reported, as the jihadists tried to repel the army.

It points to a tough battle ahead as the army aims to move from breaking the siege to driving Islamic State from its half of the city, the sort of street-by-street warfare in which the jihadists 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 Shi’ite 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 Islamic State’s 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, Islamic State 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 jihadist group specializes 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 toward Deir al-Zor, the Syrian army and its allies have been fighting Islamic State 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 jihadists’ 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 meters 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 Islamic State 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 Islamic State.

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

Islamic State 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 toward the still besieged airbase, southwards from the Brigade 137 camp and eastwards along the main highway, the commander said.

(Reporting By Angus McDowall; Editing by Angus MacSwan)

By Ellen Francis

BEIRUT (Reuters) – Syrian government forces on Tuesday reached troops besieged for years by Islamic State in the eastern city of Deir al-Zor, the militants’ last major stronghold in Syria, the army said.

Tanks and troops pressed quickly toward a government-held enclave in the city, where Islamic State has trapped thousands of civilians and Syrian soldiers since 2014. The advance has opened a land route linking that territory to the outside.

The advance into strategic prize Deir al-Zor, a city on the Euphrates river and once the center of Syria’s oil industry, is a significant victory for President Bashar al-Assad against Islamic State and another stinging blow to the group.

The group is being fought in Syria by government forces, backed by allies Iran and Russia, and separately by a U.S.-led alliance of Arab and Kurdish fighters. In Iraq, the jihadists were driven out of their Mosul stronghold earlier this year.

Islamic State still holds half of Deir al-Zor city and much of the province, however, as well as parts of its former stronghold Raqqa to the northwest, where the U.S.-backed offensive is being fought.

“Our armed forces and allies, with support from Syrian and Russian warplanes, achieved the second phase of their operations in the Syrian desert,” Syria’s military said. “They have managed to break the siege.”

State media and a war monitoring group said advancing forces had linked up with the besieged troops at a garrison on the western edge of the city.

Footage on Syrian state TV showed soldiers cheering near the garrison. State media said residents in government-held parts of the city were celebrating the advance.

The Syrian Observatory for Human Rights said a nearby military air base and three districts remained under siege by IS. Battles still raged around the city, the British-based war monitor said.

Deir al-Zor governor Mohammed Ibrahim Samra told Reuters that government troops were also pushing toward the air base.

“The forces have begun to lift the siege,” he said. “Our residents have been waiting for this moment … forces are (trying to) break the siege on the military airport as well.”

“The coming days will see the clearing of Deir al-Zor city (of militants)” and advances on nearby countryside under Islamic State control, Samra added.

Assad congratulated the troops in a statement from his office.

The army and its allies made rapid advances in recent days pushing through Islamic State lines with the help of heavy artillery and Russian air strikes.

A Russian warship in the Mediterranean sea fired cruise missiles at Islamic State positions near Deir al-Zor to boost the offensive, Russia’s defense ministry said.

ISLAMIC STATE SQUEEZED

The city has been cut off from government-held Syria since 2013, after rebel groups rose up against Assad during the first flush of Syria’s six-year war. Islamic State then overran rebel positions and encircled the army enclave and nearby air base in 2014.

The United Nations said in August it estimated 93,000 civilians were living there in “extremely difficult” conditions. During the siege, high-altitude air drops have supplied them.

Deir al-Zor lies southeast of Islamic State’s former base in Raqqa.

Hemmed in on all sides, Islamic State fighters have fallen back on footholds downstream of Deir al-Zor in towns near the Iraq border.

The Deir al-Zor gains “form an important launching pad for expanding military operations in the area,” the army said.

For Damascus, the latest advance caps months of steady progress as the army and its allies turned from victories over rebels in western Syria to push east against Islamic State.

The eastwards march has on occasion brought them into conflict with U.S.-backed forces.

Still, the rival campaigns have mostly stayed out of each other’s way, and the U.S.-led coalition has stressed it is not seeking war with Assad.

In a statement on Sunday an alliance of Iran-backed Shi’ite militias allied to Damascus, including Lebanon’s Hezbollah, accused Washington of trying to hinder the advance to Deir al-Zor.

An official in the pro-Assad alliance said senior Iranian Revolutionary Guard Corps commander Qassem Soleimani was closely monitoring fighting, a sign of Iran’s close military involvement.

(Reporting by Ellen Francis; Additional reporting by Laila Bassam in Beirut and Kinda Makieh in Damascus; Writing by John Davison; Editing by Andrew Roche and Toby Davis)