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 Islamic State 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 Islamic State 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.

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.

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