Weapons inspectors returned to Syria on Wednesday to hunt for evidence of poison gas attacks, as UN leader Ban Ki-moon pressed world powers to overcome their divisions on the conflict.

But wrangling within the Syrian opposition dealt a new blow to hopes for a peace conference to end the 30-month-old conflict that the UN says has left well over 100,000 dead.

The UN team, led by Swedish expert Ake Sellstrom, travelled in an overland convoy from Beirut to Damascus.

After determining that sarin gas was used in an August 21 attack near Damascus, the team must now try to get to up to 14 sites where allegations of chemical weapons use have been made.

The sites include Khan al-Assad near the northern city of Aleppo where a sarin attack was reported on March 19. Security for the team as it travels around Syria is a major UN concern.

During a meeting with the foreign ministers of the UN Security Council permanent members -- Britain, China, France, Russia and United States -- Ban raised a Russia-US plan to destroy President Bashar al-Assad's chemical arsenal

The UN leader called the meeting to urge the major powers to overcome deep divisions over the conflict.

Russia and the United States are negotiating a UN Security Council resolution to legally enforce the disarmament plan.

US President Barack Obama said Tuesday there had to be a "strong" resolution and French leader Francois Hollande said it must include the threat of eventual "coercive" measures.

But Russia has fiercely opposed any move to allow eventual sanctions or military force under the UN Charter.

US officials said progress was made in talks between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov but days of negotiations would still be needed.

Ban and the ministers also "exchanged views on the timing and other aspects of the peace conference to be held in Geneva," added Nesirky.

-- New Divisions Hit Syrian Opposition --

But prospects for a follow up to a peace conference held in Geneva in June last year took a blow with a new rift in the Syrian opposition.

Thirteen key Syrian Islamist groups said they did not recognise any foreign-based opposition group, including the main Syrian National Coalition, which has its headquarters in Turkey.

The Islamists include members of the main rebel Free Syrian Army and the radical Al-Qaeda-affiliated Al-Nusra Front.

Other troubles emerged when Free Syrian Army commander Methkal Albatich said in Paris that more than 100 senior rebel officers had signed a petition demanding a "boycott of any conference or discussion which involves Iran in one way or the other."

The FSA's authority has been significantly weakened, however, by the move by Islamist groups to break with the Western-backed National Coalition.

Coalition leader Ahmad Jarba had planned a press conference at the UN headquarters on Thursday but this was abruptly called off.

"It's extremely damaging," Charles Lister, an analyst at IHS Jane's Terrorism and Insurgency Centre told AFP.

The groups "represent a very significant portion of the armed opposition and the groups that have had the most strategically valuable impact."

Countries suffering the fallout from the Syria conflict have stepped up calls for international help at the UN General Assembly.

Britain on Wednesday gave an extra 100 million pounds ($160 million) to the humanitarian effort.

Total British aid for Syria now amounts to more than 500 million pounds, according to Deputy Prime Minister Nick Clegg, who announced the cash boost on the sidelines of the UN summit.

"The scale of what is unfolding in Syria is almost beyond comprehension and is certainly on a scale of humanitarian suffering that the world had not had to confront in a long period of time," Clegg told reporters.

On Tuesday, Obama announced $340 million in humanitarian aid for Syrians, bringing the US contribution to $1.3 billion.

But the UN still has a huge funding shortage for its operation to help more than two million Syrians in Lebanon, Jordan, Iraq, Turkey and Egypt and about six million are displaced inside the country.

EU aid chief Kristalina Georgieva said the UN Security Council would have to consider a binding resolution to allow humanitarian access to trapped civilians inside Syria.

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