The attack came shortly after 13 Islamist groups, led by the al-Qaeda affiliated al-Nusra Front, said they had set up a "joint military operation room."

Their heavy shelling had targeted regime-held areas in Aleppo's western districts, according to the Britain-based Observatory for Human Rights.

In retaliation, warplanes of the regime headed by President Bashar al-Assad had bombed rebel-held areas on Aleppo's northern outskirts. Different rebel factions largely control eastern areas.

The observatory said at least eight civilians had been killed and another 70 injured, alongside dozens of dead among rebels and army personnel.

"This is the heaviest attack by rebel forces Aleppo has witnessed since two years, as more than 400 shells fell on regime-held areas in Aleppo," the Observatory's Rami Abdel Rahman told German news agency dpa.

Rebels aim for supply lines

Rebel mortar shelling had been largely focused on heavily defended government compounds as the rebels sought to cut army supply lines.

Analyst Firas Abi Ali of IHS Country Risk told dpa that if Assad's forces completely lost the city, his opponents, including the attacking rebels and Islamic State militants, would likely end up fighting each other.

Since 2012, the city had "lost its strategic value from a strictly military point of view," Abi Ali said.

Fighting also in Azaz

Separately, heavy fighting in the northern Syrian town of Azaz has been heard at the Turkish border, about 50 kilometers (30 miles) away.

Sources quoted by Reuters said the clashes were between the hardline Islamic State and the joint al-Nusra and a Western-backed rebel alliance.

More than 230,000 people have been killed in Syria since the country's multi-sided civil war began in 2011 when Assad's forces repressed protests.

ipj/jr (Reuters, dpa, AFP)