Syrian rebels said Saturday they have broken a three-week government siege of second city 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 to control the provincial capital since mid-2012, transforming the former economic powerhouse into a divided, bombed-out city.

Opposition fighters, Islamists and jihadists have waged fierce assaults since July 31 to end the siege by government forces of some 250,000 people in eastern Aleppo.

On Saturday, rebels successfully broke the siege by opening a new route into the city from the southwest, opposition officials said.

"Rebels break Aleppo's siege," tweeted the Istanbul-based opposition National Coalition.

Islamist faction Ahrar al-Sham said rebels had seized control of Ramussa on the southwestern edges of the city and "opened the route to Aleppo".

But state television said late Saturday the army had regained control of several key points in a military complex that rebels overran earlier.

"Our armed forces took control of the part of the artillery school that had been seized by terrorists... while (rebel) groups withdrew from points in the armament school," it said, citing a military source.

- 'A good omen' -

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 said residents were on the streets and firing celebratory shots 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.

An AFP photographer said the first truck of vegetables in a month entered eastern Aleppo via Ramussa on Saturday.

The tomatoes and potatoes were destined for markets in rebel-held districts, which had seen prices skyrocket since the siege began.

The Syrian Observatory for Human Rights confirmed that rebels in Sheikh Saeed district in the south of Aleppo had linked up with other insurgents in part of Ramussa but stressed that the rest of that area was still in regime hands.

The Britain-based monitor said rebels and jihadists had expanded their control late Saturday to seize the air force technical school south of Aleppo.

"The western districts of Aleppo are now besieged. There are no safe routes for civilians in government-held districts to use to get into or out of the city," said Observatory head Rami Abdel Rahman.

The monitor said more than 700 rebels and government fighters had been killed since the offensive began on Sunday -- 200 on Saturday alone.

At least 130 civilians had been killed, among them seven on Saturday in rebel shelling of the regime-held neighbourhood of Hamdaniyeh, the Observatory said.

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

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

- IS loses Manbij -

Fearful residents of western districts watched the news on television screens in street cafes.

"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.

"We are thinking about how to leave," he told AFP.

Also on Saturday, the US-backed Syrian Democratic Forces alliance defeated IS in Manbij.

The Observatory said the SDF "took control of Manbij... and are combing the city in search of the last remaining jihadists".

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

Backed by the US-led air coalition, the SDF offensive began on May 31 and it entered the town less than a month later, in an assault slowed by jihadist suicide attackers and car bombs.

The Manbij Military Council -- a key component of the SDF -- said fighting was continuing near the town centre, however.

"We are in control of 90 percent of Manbij," spokesman Sherfan Darwish said.

In the northwestern province of Idlib bordering Turkey, raids by either regime or Russian warplanes on Saturday near a hospital in Milis killed at least 10 civilians, the Observatory said.

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