Saudi Arabia, a powerful supporter of the Syrian rebels, called on Saturday for an immediate end to what it said were war crimes being committed by forces loyal to President Bashar al-Assad as they retake rebel-held parts of Aleppo.

"It is by far the worst humanitarian tragedy of the beginning of the 21st century unfolding before the international community's eyes," state news agency SPA quoted a foreign ministry official as saying.

"The horrific massacres perpetrated in Aleppo ... have amounted to war crimes against humanity," the official said.

The city had been divided between government and rebel areas in the nearly six-year-old civil war, but an advance by the Syrian army and its allies that began in mid-November deprived the insurgents of most of their territory in a matter of weeks.

An operation to evacuate fighters and civilians from the last opposition-held area was suspended on Friday, its second day, after pro-government militias demanded that wounded people also be brought out of villages where they are under siege from the rebels.

Officials from both sides said on Saturday that a new deal was being negotiated to complete the evacuation.

Saudi Arabia accuses regional rival Iran of meddling in the affairs of other states, including Syria, to expand its influence in the Arab world.

The Saudi source said Riyadh was in contact with regional and international powers "emphasizing the importance of taking immediate action to stop the carnage" in Aleppo.