The UN Security Council will hold an emergency meeting tomorrow on Syria after a UN envoy warned that eastern Aleppo may be totally destroyed in the next few months by the Russian and Syrian air campaign.

Russia requested the meeting to hear from UN envoy Staffan de Mistura, who will brief the council via video conference from Geneva at 1400 GMT, diplomats said.

Mr De Mistura earlier took aim at Russia, suggesting that Moscow was indiscriminately bombing a city with hundreds of thousands of civilians to flush out just a few hundred jihadists.

"We are talking about 900 people, basically, who are becoming the main reason for which there is 275,000 people actually being attacked," he said.

Would this, he asked, be the excuse for "the destruction of the city?"

"In maximum two months, two-and-a-half months, the city of eastern Aleppo may be totally destroyed," he told reporters.

The envoy urged fighters from the former Al-Nusra Front - which renamed itself Fateh al-Sham Front after breaking with Al-Qaeda - to leave Aleppo under a deal to halt the regime's attacks on the city.

"If you decide to leave with dignity ... I am personally ready to physically accompany you," Mr de Mistura said.

Security Council members were discussing a French-drafted UN resolution calling for a ceasefire in Aleppo.

After holding talks in Moscow on the proposal, French Foreign Minister Jean-Marc Ayrault will head to Washington tomorrow to discuss the measure, which calls for ending all flights over Aleppo.

French Ambassador Francois Delattre told reporters that "we have a strong determination to go to a vote" on the draft resolution, which could lead Russia to use its veto power as a permanent member of the council.

Guterres vows to serve 'the most vulnerable'

Earlier, Antonio Guterres, the man set to become the next United Nations secretary-general, vowed to serve "the most vulnerable" when he takes up the post.

"To describe what I feel at the present moment two words are sufficient: gratitude and humility," the former Portuguese prime minister said in an address at the foreign ministry in Lisbon.

"Gratitude but also humility. Humility when facing the dramatic problems of today's world and humility that is needed to serve, and especially to serve those that are most vulnerable," he added.

"The victims of conflict, of terrorism, the victims of the violation of rights, the victims of poverty and injustices."

It was his first public comment since the UN Security Council earlier unanimously backed Mr Guterres, who was chief of the UN's refugee agency for a decade, to be the next secretary-general.

A vote by the UN General Assembly's 193 member states to endorse him as successor to Ban Ki-moon is expected next week.

The unanimous Security Council backing for Mr Guterres for a five-year term from 1 January followed an informal vote yesterday during which 13 of the 15 members supported his candidacy and none of the five veto-holding powers blocked him.

"I was moved when I saw the Security Council able to decide in unity and consensus and to decide in a very quick way," Mr Guterres said in an address repeated in English, French, Portuguese and Spanish.

"And I hope this represents a symbolic moment, a moment in which the Security Council enhances its capacity to act in unity and consensus creating the conditions to decide timely in relation to the dramatic problems of out time."

Mr Guterres, who served as Portugal's prime minister from 1995 to 2002, won the number-one spot in all of the informal votes held by the Security Council.

The 67-year-old, who will be the first former head of government to lead the United Nations, has pledged to revamp the global diplomatic body to boost its peacemaking efforts and promote human rights.