BEIRUT — Syrian government forces have broken a long siege on the strategically situated prison in the northern city of Aleppo, government and pro-opposition groups said Thursday.

The advance of tanks and troops into the sprawling prison complex on the northeastern edge of Aleppo is the latest victory for government forces ahead of the presidential election scheduled for June 3.

The government is keen to project an image of mounting battlefield victories in the approach to the election, which is widely expected to result in another seven-year term for President Bashar Assad.

Forces loyal to Assad have fought off a three-year uprising and have taken the offensive on several fronts in Syria, inflicting strategic defeats on rebels backed by the United States and its allies.

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Earlier this month, the Syrian military recaptured the Old City of Homs, a longtime opposition bastion in the center of the nation. Beleaguered rebels agreed to evacuate the area in a deal between the two sides.

In Aleppo, analysts say, Syrian forces may be trying to re-create the Homs scenario by pushing rebels back, cutting off their supply lines and forcing a surrender or retreat.

But the military, overstretched as it fights on numerous fronts, may face a long battle before its troops can encircle rebel-held strongholds in Aleppo. Rebels control much of the city’s east and several suburbs and routes leading in and out of the city.

The army has made some territorial gains in the area but has faced fierce resistance from various rebel brigades, including forces of Al Nusra Front, the al-Qaida franchise in Syria.

“The fall of the prison does not lead to a siege on Aleppo,” an opposition activist in the city who goes by the nickname Abu Yazeed Al-Halabi said via Skype. “But the regime is working on completing the perimeter around Aleppo and besieging it as it did in Homs.”

The move on the Aleppo prison came a few hours before Russia and China vetoed a U.N. Security Council Resolution referring the Syrian crisis to the International Criminal Court for investigation of possible war crimes. It was the fourth time the two countries have used their veto power as permanent council members to deflect action against Assad’s government.

Both the state-run news media and a pro-opposition monitoring group reported Thursday that the military had been able to enter the prison grounds, which had been under rebel siege for 13 months.

The government had reportedly been forced to airlift supplies into the facility as its forces fought off rebel thrusts, led on several occasions by bomb-laden vehicles driven by suicide bombers.

As many as 4,000 prisoners were said to be in the prison. There have been unconfirmed reports that some of them were killed during the periodic battles and that others have died of hunger-related ailments.

On Thursday, photos posted on Twitter purported to show haggard prisoners behind bars welcoming army troops.

In a statement, the army called the advance “highly important” as “it serves to tighten the noose on residual terrorist groups in the eastern and northeastern outskirts of Aleppo city and cuts off supply lines that terrorist gangs were using.”

Syrian officials routinely refer to the rebels as “terrorists.”

Aleppo, once Syria’s commercial hub and its most populous city, has been divided between government and rebel forces for almost two years. Large swaths of the city have been destroyed and much of the population has fled amid clashes and heavy government bombardment.

Also Thursday, the global chemical-weapons watchdog said the last 100 metric tons of Syria’s declared stockpile of precursors for poison gas and nerve agents have been packed and are ready for transport, though the Assad government said it’s too risky to move them.

Syria is well behind schedule in removing its stockpile of some 1,300 tons of chemicals that it declared to the Organization for the Prohibition of Chemical Weapons last year. All the chemicals were supposed to have been destroyed by the end of June, but that deadline appears out of reach. The U.S. says it needs 60 days to destroy hundreds of tons of highly toxic chemicals on board a specially equipped ship.

Material from The Associated Press is included in this report.