The new UN-Arab League envoy on Syria, Lakhdar Brahimi, has given a deeply pessimistic view of the task ahead of him, as he takes up his new post.

Speaking to the BBC, the Algerian described his mission as "nearly impossible".

Brahimi, a seasoned UN diplomat, was appointed after his predecessor, Kofi Annan, resigned, saying he no longer saw a way to fulfill his mission. Fighting in Syria has escalated, despite a truce mediated by Annan.

Activists say 20,000 people have died since the uprising against the Syrian government began last March. On Sunday, Sept 2, the pro-rebel Syrian Observatory for Human Rights said more than 5,000 people were killed in August alone.

"I'm coming into this job with my eyes open, and no illusions," Mr Brahimi told the BBC. "I know how difficult it is - how nearly impossible. I can't say impossible - [it is] nearly impossible.”

Brahimi admitted some trepidation about his new mission, saying he could understand those frustrated with the lack of international action in Syria.

"I'm scared of the weight of responsibility. People are already saying people are dying and what are you doing? And we are not doing much. That in itself is a terrible weight."

He said he would keep Mr Annan's six-point peace plan - now seen by many as irrelevant - in his "tool box" for possible adaptation, but admitted he "had ideas, but no plan yet", apart from talking to as many people as possible.

Addressing the Syrian government, he said the need for political change in Syria was "fundamental and urgent", but - as he has previously - refused to be drawn on whether President Assad should step down, as the opposition and several Western leaders are demanding.

"Change cannot be cosmetic," he said. "There will be a new order but I do not know who will be the people in the order. That's for Syrians to decide."

He also sought to keep a distance between himself and the rebels, who have criticized him for his cautious stance.

"Please remember I am not joining your movement," he said. "I am working for two international organizations, the United Nations and the Arab League, and I do not speak the same language as you."