BEIRUT - Syrian rebels claimed on Saturday they broke a three-week government siege of second city of Aleppo, turning the tables on Russian-backed regime forces who are now on the defensive.

To the northeast, a Western-backed alliance of Arab and Kurdish fighters scored a major victory against the Islamic State group in the town of Manbij after a fierce two-month battle.

The developments have rocked the key northern province of Aleppo, a microcosm of Syria’s chaotic multi-front war that has killed more than 280,000 people. Rebel and regime forces have fought for control of the provincial capital of the same name since mid-2012, transforming the former economic powerhouse into a divided, bombed-out city.

Opposition fighters, Islamists and militants have waged fierce assaults since July 31 to end the siege of some 250,000 people in Aleppo’s eastern districts by government forces. On Saturday, rebel groups successfully broke the siege by opening a new route into the city from the southwest, opposition officials said. “Rebels break Aleppo’s siege,” wrote the Istanbul-based National Coalition on Twitter.

The Islamist faction Ahrar al-Sham said on Twitter that rebels had seized control of Ramussa on the southwestern edges of the city and thereby “opened the route to Aleppo”.

Al-Manar, the television station of the Lebanese group Hezbollah which has men fighting alongside Syrian government forces, denied that Ramussa had fallen or that the siege had been broken. Riad Hijab, head of the broad opposition body the High Negotiations Committee, tweeted, “The liberation of Ramussa and the breaking of the siege are a good omen for Syria’s revolution.”

An AFP journalist in eastern Aleppo city said joyful residents were out on the streets and shooting celebratory gunfire into the air.

“Days ago, I was only thinking about how to get a bite to eat,” said Ahmad Adna, a 46-year-old resident of eastern Aleppo. “Now I’m more optimistic after the (rebel) Army of Conquest’s advance. I hope today will be the last day of the siege.”

Earlier Saturday, rebels and allied militants captured swathes of a military academy south of the city, according to the Syrian Observatory for Human Rights. The monitor said more than 500 rebels and government fighters had been killed in the offensive since it was launched on Sunday, as well as 130 civilians.

They included at least seven civilians killed on Saturday in rebel shelling of the regime-held neighbourhood of Hamdaniyeh, according to both the Observatory and Syrian state news agency SANA.

The former Al-Nusra Front - renamed the Fateh al-Sham Front after breaking from Al-Qaeda - said on Saturday that rebels pushing out from inside Aleppo city had linked up with those on the outskirts.

Drone footage posted by the group online showed a series of explosions on the edges of Aleppo, followed by massive columns of billowing black smoke.

Pictures obtained by AFP show a crumpled body, reportedly of a regime fighter, lying next to artillery pieces lined up in a building newly captured by militants.

Abdel Rahman said the advance had left the regime forces “in a very difficult position despite Russian air support”. “This is an existential battle. Whoever wins it will win Aleppo,” he said.

State media said the army launched a counter-offensive in the afternoon and deployed reinforcements to take on “thousands of terrorist fighters”.

An army officer told state television that troops advanced in the areas seized by rebels and were inflicting “heavy losses” on them.

“Of course I have faith in the army, but I can’t help being scared. Food is already getting more expensive and the coming days risk being very difficult,” said a 34-year-old resident of a government-held western quarter of Aleppo. “We are thinking about how to leave,” he told AFP. Also on Saturday, the US-backed Syrian Democratic Forces alliance handed a major defeat to IS in Manbij. The Britain-based Observatory said the SDF “took control of Manbij... and are combing the city in search of the last remaining militants”.

Meanwhile, An airstrike on a hospital in northwestern Syria on Saturday killed 10 people, including children, the Syrian Observatory for Human Rights said.

The hospital, in the town of Meles, is about 15 km from Idlib city in rebel-held Idlib province.

Syrian government and allied Russian military planes operate in Syria but it is not known which aircraft carried out the strike, the Observatory said.

Humanitarian groups have repeatedly called for a halt to strikes on medical facilities. Syrian opposition groups say Syrian and Russian forces deliberately target medical buildings.

Medical charity the Syrian American Medical Society (SAMS) said that July was the worst month for attacks on healthcare centres since the beginning of Syria's five-year-old conflict.

"There were 43 attacks on healthcare facilities in Syria in July - more than one attack every day," a SAMS statement said.

The Syrian government has also accused rebels of hitting medical targets in the city of Aleppo.

Manbij had been a key transit point along IS’s supply route from the Turkish border to Raqa, the de facto capital of its self-styled “caliphate”.

With backing from the US-led air coalition, the SDF launched its offensive on May 31 and entered the town less than a month later, but the assault was slowed by a militant fightback using suicide attackers and car bombs. The Manbij Military Council - a key component of the SDF - said fighting was still ongoing.

“The battles are continuing near the centre of the town. We are in control of 90 percent of Manbij,” spokesman Sherfan Darwish said.

Syria’s conflict first erupted in March 2011 with anti-government protests but has since evolved into a full-fledged war largely dominated by militant groups.