Veteran Algerian diplomat Lakhdar Brahimi is expected to replace Kofi Annan as the UN-Arab League joint special envoy for Syria barring a last-minute change, diplomats said today.

The former Algerian foreign minister, who has a long history as a diplomatic troubleshooter, will have his work cut out for him in Syria, where President Bashar al-Assad is using his security forces to try to crush a 17-month-old pro-democracy rebellion.

Annan, a former UN secretary-general and Nobel Peace Prize laureate, said last week he would step down as the special envoy because he was unable to do his job with the UN Security Council hopelessly deadlocked over Syria.

Brahimi's appointment could be announced as early as next week but the diplomats, who spoke on condition of anonymity, said there are sometimes last-minute changes if a key government has concerns about the choice or the candidate has misgivings.

Brahimi, 78, has served as a UN special envoy in a series of challenging circumstances, including in Iraq after the US invasion that toppled Saddam Hussein, in Afghanistan both before and after the end of Taleban rule and in South Africa as it emerged from the apartheid era.

Syria, however, may present an unusually vexing assignment, in part because international action to try to end the violence has been stymied by the disagreements between the five veto-holding permanent members of the UN Security Council.

While the council united in April to approve the deployment of 300 monitors to Syria to observe a failed ceasefire as part of Annan's peace plan, Russia and China vetoed three other resolutions that criticised Syria and threatened sanctions against Damascus.

In announcing his resignation, Annan explicitly blamed "finger-pointing and name-calling" at the Security Council for his decision to quit, but suggested his successor may have better luck.

Assad's forces have killed more than 15,000 people since March of 2011 in a sustained effort to end the anti-government rebellion, some Western leaders say. Damascus says the rebels have killed several thousand members of its security forces.

Assad has suffered a series of blows in recent weeks, including the defection of his prime minister, Riyad Hijab, on Monday and the assassination of four of his top security officials last month.

He named a new prime minister, Wael al-Halki, overnight (NZT) as government forces pushed rebels back from a strategic district in Aleppo, Syria's commercial hub and largest city.

In accepting Annan's resignation, UN Secretary-General Ban Ki-moon thanked him for having taken on "this most difficult and potentially thankless of assignments".

A spokesman for Ban, who is expected to formally name Annan's successor, was not immediately available for comment.

- Reuters