BEIRUT — Backed by Russian and Iranian firepower, President Bashar Assad’s forces reached besieged troops Tuesday at a garrison in Syria’s eastern city of Deir el-Zour, breaking a nearly three-year blockade by Islamic State militants and marking a significant advance against the extremists.

Re-entering Deir el-Zour would bring the Syrian forces and their allied Iranian-backed militias a step closer to controlling the oil-rich eastern province and its capital bordering Iraq. Such a move would also boost Tehran’s growing influence in the area.

Assad congratulated his troops on breaking the siege as a “resounding victory” against extremism and vowed to forge ahead until “the last inch” of Syrian territory is liberated. Russian President Vladimir Putin, whose military had fired cruise missiles at IS targets near the city, called it a “strategically important victory over terrorists.”

Assad critics called it an alarming development.

“It opens the whole province for Iran and its agents there,” said Mozahem al-Salloum, an opposition activist from the city who has been in exile since 2013.

Iranian- and Lebanese-backed militias form the core of the pro-Assad forces advancing on Deir el-Zour, he noted.

Iran has been seeking to secure a land corridor from its territory, through Iraq, to the Mediterranean to give it unhindered access to its allies in Damascus and Beirut. Control of Deir el-Zour is a major boost for that plan.

By nightfall, activists said the IS militants had counterattacked with four suicide assaults near where the Syrian troops had linked up.

The advance by the Syrian troops was celebrated as a possible relief for the tens of thousands of civilians trapped in the four government-controlled neighborhoods that have been surrounded by the extremists since 2015. The Syrian government estimates about 70,000 people have survived on erratic air drops of food and supplies during the siege, which was a major embarrassment to Assad.

Activists noted, however, that the new access road could not yet be used for delivering humanitarian assistance because it was still under attack from IS.

Deir el-Zour, Syria’s largest eastern city, has been divided into government- and IS-controlled parts since 2015. The province is held by the extremists and it is where they are expected to fight their last battles. They have lost all other major cities, including the Iraqi cities of Mosul and Tal Afar. U.S.-backed Syrian troops are bearing down on Raqqa, the group’s self-proclaimed capital that is northwest of Deir el-Zour.

It could take weeks, if not months, for Assad’s forces to retake Deir el-Zour from the militant group, which controls about 60 percent of its neighborhoods.

Government troops have been advancing on the Euphrates River Valley city for weeks, carrying out a multipronged offensive from the northwest, west and southwest. They took control of a strategic mountain to the northwest of the city last week, giving them superior firepower into the area.

This enabled Tuesday’s advance, when pro-government forces reached a military base of the 137th Brigade on the outskirts that has been surrounded for months.

Rami Abdurrahman, the head of the Syrian Observatory for Human Rights, said about two dozen soldiers first advanced toward the base to open and secure the road filled with land mines. They were followed by two dozen others who were able to link up with soldiers inside the base.

Salloum, who operates the activist-run Hammurabi Justice News network, said government and allied troops remain engaged with IS militants on the southern flank, where the militants have carried a counteroffensive near the town of al-Shola.

The Russian Defense Ministry said one of its frigates in the Mediterranean unleashed cruise missiles early Tuesday toward IS targets near the city. They fired at a fortified area around al-Shola, where most of the militants are believed to be from Russia and former Soviet republics.

The Defense Ministry said drone footage showed that the missiles destroyed a communications center, command centers, ammunition depots and a repair shop for armored vehicles, as well as killing an unspecified number of fighters. But the militants still control al-Shola, Abdurrahman said.

The Syrian army command said reaching Deir el-Zour marks “a strategic turn in the war against terrorism,” and that the city will be used “to expand military operations in the region.”

The militants “did not give up easily and used lots of suicide car bombs yesterday, but could not resist much,” said opposition activist Omar Abu Laila, who lives in Europe but is from Deir el-Zour and is in contact with people there.

A former opposition fighter who used to live in the city welcomed the lifting of the siege but expressed concern over the fate of civilians in IS-controlled areas. He spoke on condition of anonymity because of fears for the safety of his family, which still lives in the IS-controlled neighborhoods.

The offensive was led by Gen. Suheil al-Hassan, who is known as “the Tiger.” He has been behind other recent victories, including the capture of eastern parts of the northern city of Aleppo in December, the government’s biggest victory since the Syrian conflict began in 2011.


Associated Press writers Bassem Mroue in Beirut, Nataliya Vasilyeva in Moscow and Albert Aji in Damascus, Syria, contributed.