Dozens of the world's top diplomats will gather this weekend to turn up pressure on Syria to abide by a peace plan that could finally end violence there.

Representatives of 71 countries are expected at the "Friends of Syria" conference in Istanbul to come up with measures to ward off a Damascus backtrack on Kofi Annan's peace initiative.

The plan was approved by Syrian President Bashar al-Assad on Tuesday.

"The main objective of this conference is to increase the pressure on the Syrian regime to end the bloody repression" said a Turkish official.

However, Mr Annan, the UN-Arab League envoy, will be absent due to a presentation he will deliver for the UN on Monday.

The conference, like its predecessor in Tunis in late February, will again lack participation from Russia and China, the two supporters of an Assad regime now shunned by its Arabic neighbours and most of its former Western allies.

Western and Arabic representatives will evaluate whether to hit Syria with more sanctions while providing broader aid to its opposition.

"We're going to be continuing to look at what more we can do on the sanctions side to pressure the regime," US State Department spokeswoman Victoria Nuland said of the conference in Istanbul.

"And obviously, we'll all be comparing notes on how we can support Kofi Annan, particularly on the important point of getting Assad to meet the commitment that he's made," she said.

The US State Department earlier announced that Secretary of State Hillary Clinton will attend the meeting.

The conference does not aim to parallel Mr Annan's initiative, but rather to act as an endorsement for it, a European diplomatic source said.

"The EU wants to accompany the diplomatic mission by Annan and respond positively if he needs support," he noted.