(Original post by BBC News Website)
The UN-Arab League joint special envoy to Syria, Kofi Annan has announced he is leaving his post.
In a news conference, he said the Syrian people "desperately need action" but criticised the UN Security Council for "finger-pointing and name-calling".
Mr Annan authored a six-point peace plan for Syria which was intended to bring an end to the fighting.
But the plan was never fully adhered to by either side and the violence has continued to escalate.
UN Secretary General Ban Ki-moon said it was "with deep regret" that he announced Mr Annan would not renew his mandate when it expires at the end of August.
Syrian state television reported that the foreign ministry had also expressed regret at the announcement.
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Syria can still be saved from the worst calamity - if the international community can show the courage and leadership necessary”
Speaking to reporters in Geneva, Mr Annan said the increasing militarisation of the Syrian conflict and the "clear lack of unity" in the Security Council had "fundamentally changed the circumstances for the effective exercise of my role".
He said the problems were "compounded by the disunity of the international community".
"When the Syrian people desperately need action, there continues to be finger-pointing and name-calling in the Security Council," he said.
"It is impossible for me or anyone to compel the Syrian government, and also the opposition, to take the steps to bring about the political process.
"Syria can still be saved from the worst calamity - if the international community can show the courage and leadership necessary to compromise on their partial interests for the sake of the Syrian people - for the men, women and children who have already suffered far too much."
The veto-wielding members of the UN Security Council have failed to agree on what action to take on Syria.
Russia and China have vetoed resolutions on the crisis three times, citing opposition to any action which might be seen as regime change imposed from outside.
Mr Annan said he did not rule out someone taking over the mediator's role from him, but said a successor might choose another path.
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Annan's six-point plan
1. Syrian-led political process to address the aspirations and concerns of the Syrian people
2. End to violence by all sides; army troops to stop using heavy weapons and withdraw to barracks
3. Parties to allow humanitarian aid
4. Authorities to free political detainees
5. Authorities to ensure freedom of movement for journalists
6. Authorities to allow peaceful demonstrations
Syria: Guide to the conflict
He said the focus remained on political transition, as President Bashar al-Assad "will have to leave sooner or later".
In his statement, Mr Ban said he was in discussion with the Arab League to find a successor to "carry on this crucial peacemaking effort".
He said Mr Annan deserved "profound admiration" for the way he had tackled "this most difficult and potentially thankless of assignments" and that he remained convinced bloodshed would only bring "deeper suffering to the country and greater peril to the region".
Mr Ban said the Annan plan remained the "best hope for the people of Syria" but that the "persistent divisions" in the UN Security Council "have themselves become an obstacle to diplomacy, making the work of any mediator vastly more difficult".
Russia said it regretted that Mr Annan had chosen to stand down.
The Russian ambassador to the UN, Vitaly Churkin, said Moscow had always supported Mr Annan's work and that it hoped Mr Annan's final month in the role "is going to be used as effectively as possible under these very difficult circumstances".
US spokesman Jay Carney blamed Russia and China for the resignation, saying it highlighted their failure at the UN to "support meaningful resolutions against Assad that would have held Assad accountable".
The BBC's Jim Muir, who is monitoring events in Syria from neighbouring Lebanon, said Mr Annan's decision to step down is clear recognition that the political process has failed, and that Syria's fate will be decided by events on the ground.
It is hard to imagine a figure with anything approaching the stature and profile of Mr Annan taking over the task, when the prospects for success are currently negligible, our correspondent adds.
Mr Annan took up his post in February. His internationally backed peace plan called for an end to the use of heavy weaponry, the free passage of aid, freedom of media and demonstration and for a Syrian-led political process to address the aspirations and concerns of the Syrian people.
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In pictures: Aleppo fighting
It was supposed to come into effect in mid-April, but government forces continued to shell opposition strongholds and the opposition forces never fully committed to it.
Activists estimate some 20,000 people have died since anti-government protests erupted against President Assad in March last year. Tens of thousands of people have also fled the country.
Earlier on Thursday, rebel fighters in Syria's second city, Aleppo, attacked an army base using a tank they had seized from the military.
Aleppo has been the scene of fierce fighting in the past week as the pro-government troops attempt to stop rebels from taking control of the city.
In the capital, Damascus, government forces launched two operations to root out rebel activists on Wednesday, killing at least 70, the opposition said.
Troops reportedly went from house to house demanding to see people's papers. Activists say the soldiers summarily executed many of their victims.
India is saddened the diplomatic effort to solve this crisis has been damaged by the departure of Mr Annan. India believes only diplomacy can end the conflict and hopes this minor setback can soon be overcome.
Georgia would also add to the messages of thanks to Kofi Annan, for all his efforts in trying to find a diplomatic solution to this conflict. Georgia is not surprised that the Annan plan has failed, given the fierce opposition to it in the SC by Russia. Georgia would like to reiterate the conflict in Syria can only be ended diplomatically, a military solution is not an option. Al-assad has lost all legitimacy. The people of Syria have spoken and we support their decision.
San Marino regrets the decision made by Mr Annan, and thanks him for his valiant effort to support the peace process in Syria. San Marino hopes that this is but a minor blip in the process, and peace talks can continue momentarily.