The wide-scale attack on regime forces in Aleppo started on Friday morning, a Britain-based monitoring group said.

The Syrian Observatory for Human Rights reported that the assault targeted southwestern parts of Aleppo, with rebels setting off three car bombs.

"Some 150 shells also rained down on areas controlled by the regime in western Aleppo," said Rami Abdel-Rahman, the Observatory's head.

At least 15 people were killed and 100 injured in the rebel shelling on the western section of Aleppo, he added.

Pledge to undo regime advance

Abu Yusef Muhajir, a military commander and spokesman for the Ahrar al-Sham rebel group, said the operation involved an array of opposition factions. Rebel groups announced "the start of the battle to break the siege of Aleppo, which will end the regime occupation of western Aleppo and break the siege on the people trapped inside," he said.

Rebel sources said they had taken control of several positions from government forces, including a checkpoint at a factory. But a Syrian military source said the army had thwarted what he called "an extensive attack" on the city's southwestern suburbs. 

The offensive is aimed at weakening the government's siege of the opposition-held enclave in the divided city. Regime forces had intensified their own attempt to capture rebel-held areas in recent weeks.

'We will liberate Aleppo'

During talks with his Russian and Iranian counterparts in Moscow on Friday, Syrian Foreign Minister Walid al-Moallem vowed that government troops would not lose control of the city. "We will not reduce our forces in the fight against terrorism. We will liberate Aleppo from terrorists and unify the city," al-Moallem said, according to the Interfax news agency.

He added that the regime was willing to observe a new ceasefire for Aleppo if foreign powers backing the rebels - including the US, Turkey and Gulf states - could guarantee that the opportunity would be used to allow civilians to flee the city. 

Moscow, which provides air support to Syria's army, has suspended airstrikes on Aleppo for a number of days following an international outcry over the humanitarian crisis. The Russian Defense Ministry on Friday sought permission from President Vladimir Putin to restart the campaign, citing an upsurge in militant activity, but the president denied the request saying it was "not the right time" to resume airstrikes in Aleppo.

At Friday's three-way meeting, Russian Foreign Minister Sergey Lavrov said the countries had agreed to intensify the fight against terrorism in Syria, as well as to increase efforts to improve civilians' access to aid. 

The definition of which groups qualify as terrorists, however, has long divided Moscow and Washington, which supports some rebel groups in Syria.

Divided city

Once Syria's commercial hub, Aleppo has been divided mainly into the government-controlled west and rebel-held east since fighting erupted in mid-2012.

An estimated 300,000 people have been under siege by government forces since July in the eastern part of Aleppo. There are concerns that basic food, water and medical supplies are running out.

Government forces, backed by Russia, have stepped up attacks in Aleppo over the past month in an attempt to overtake the rebel-held area.

The operation has killed hundreds of civilians and destroyed infrastructure, including hospitals, prompting international condemnation.

mm,nm/kl (AFP, AP, dpa)