In a recent interview with BBC, the veteran Algerian diplomat described his mission as “nearly impossible”.
Lakhdar Brahimi was appointed after his predecessor, Kofi Annan, resigned, saying he no longer saw a way to fulfill his mission after his peace plan failed to achieve a meaningful ceasefire.
Fighting in Syria has been escalating.
In the latest – still unconfirmed – incident, opposition activists say a warplane killed as many as 18 people in a single strike in Aleppo province.
Activists say 20,000 people have died since the uprising against the Syrian government began last March.
On Sunday, the pro-rebel Syrian Observatory for Human Rights said more than 5,000 people were killed in August alone.
The conflict has increasingly come to resemble a full-scale civil war, forcing an estimated one million Syrians from their homes.
Last month, the United Nations wound up the observer mission that had been tasked with monitoring the ceasefire in Syria under the six-point peace plan negotiated by Mr Annan.
“I’m coming into this job with my eyes open, and no illusions,” said Lakhdar Brahimi.
“I know how difficult it is – how nearly impossible. I can’t say impossible – [it is] nearly impossible.”
With few people believing that Lakhdar Brahimi can do much, it seems he sees his job as keeping expectations low.
Lakhdar Brahimi is expected to visit Syria and meet President Bashar al-Assad on 8 September.
The spokesman for the Syrian foreign ministry, Jihad Makdissi, said Syria would “give Brahimi all that he needs to make his mission a success for the interest of the country”.
A former Algerian foreign minister, Lakhdar Brahimi has also held a series of key UN appointments, including that of UN envoy to Afghanistan and mediator of the peace deal that ended the Lebanese civil war.
Analysts say he has a formidable reputation at the UN and his appointment has been widely welcomed.
But Lakhdar Brahimi admitted to 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.”
Lakhdar Brahimi said he had so far failed to see “any cracks” in the “brick wall” that had defeated Mr Annan – an “intransigent” Syrian government, escalating rebel violence and a paralyzed UN Security Council, where China and Russia have vetoed several resolutions aimed at putting pressure on Damascus.
He said he would keep Kofi 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 Bashar al-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.”
Lakhdar Brahimi’s comments came after another day of violence inside Syria on Sunday.
In Damascus, an explosion hit a district where major military and security compounds are located, reports say.
State TV described the blast – involving two bombs – as “terrorism” and said four people had been lightly injured.
Activists said more than 100 people were killed on Sunday, at least 25 of them in the village of al-Fan near Hama, when it was stormed by government forces.
Many of the 25, all men, were killed by army shelling, activist groups said, but they named at least nine who they said had been summarily executed in their homes by government forces or militia.
State television said security forces had attacked what it called an armed terrorist group in the area, killing all of them.
Rebels and government forces have been involved in a fierce battle for Damascus since July.
The building affected was a base for officers guarding the joint chiefs of staff offices nearby but was empty at the time, officials said.
Bomb attacks in Damascus and the largest city, Aleppo, have become increasingly frequent in recent months, with the authorities often blaming them on “armed terrorist gangs”.
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