The United States is set to call for tough UN sanctions against President Bashar al-Assad and his inner circle at a "Friends of Syria" meeting of over 100 Western and Arab nations in Paris Friday.

But despite the estimated killing of more than 16,500 people in the 16-month uprising, key Syria allies Russia and China -- both with UN veto rights -- are not attending.

The Paris meeting follows a gathering in Tunis and another in Istanbul, both of which called in vain for tougher action against Assad's government.

China did not attend either of those meetings in which the United States, France, Britain, Germany as well as Saudi Arabia and Qatar led a group of more than 60 members, including most EU states and many Arab League nations.

UN Security Council members the United States and France will lead calls at the Paris talks for tough new UN sanctions to be imposed on Assad and his inner circle.

Speaking as US Secretary of State Hillary Clinton flew to Paris for the meeting, one US official said it was time "to put this all together under a Security Council resolution that increases the pressure on Assad, including having real consequences," such as economic sanctions.

"We, and we believe most of the countries represented in Paris, think that has to include Chapter 7 economic sanctions on Assad," said the official aboard Clinton's plane who requested anonymity, referring to a clause within the UN charter.

"Many of the countries in Paris already have those sanctions but globalising them will be very important. That is the argument that we will continue to make to Russia and China."

"There's already a lot of work being done in New York in terms of thinking through what this resolution might look like," said another US official.

"The idea is to... go right away to New York, there's no wait-time. How long it will take for this all to be agreed, it's hard to anticipate. But the work is already under way and the focus will be in New York next week as soon as we finish in Paris basically."

Chapter 7 of the UN charter, which allows for sanctions ranging from economic measures to an arms embargo, and if necessary military force, was last used against Libya in 2011. But it could be highly controversial at the UN Security Council, given Russia and China's veto powers.

French President Francois Hollande will open Friday's talks, which will also be attended by the British and German foreign ministers.

French Foreign Minister Laurent Fabius told Le Parisien newspaper on Friday that "several things" would be decided at the talks, including a call for Chapter 7 backing.

"Broadening the sanctions on the Syrian regime, supporting the opposition by supplying means of communication, supporting humanitarian networks and calling for the text adopted last week to be sent to the UN Security Council to be made obligatory with the framework of Chapter 7 of the UN charter," he said.

China backed Russia at talks in Geneva last weekend, insisting that Syrians must decide how the transition should occur, rather than allowing others to dictate their fate, and did not rule out Assad remaining in power in some form.

The West insists that Assad should not be part of any new unity government and the Syrian opposition rejected the Geneva talks as making concessions to Damascus under pressure from Russia.

Russian Foreign Minister Sergei Lavrov confirmed on Thursday that Western nations had asked Moscow to offer Assad asylum and that Russia had thought it was a joke.

The diplomatic source nevertheless insisted that "Russian political and security circles are changing their position, they agree that the situation is deteriorating... even if they continue to defend Moscow's policies."

Monzer Makhous, the opposition Syrian National Council's Europe coordinator, said he also wanted the Paris meeting to call for the United Nations to force Damascus to apply Annan's peace plan.

"We demand the Syrian file be sent to the Security Council with a Chapter 7 resolution to force the regime to implement the Annan plan," Makhous told AFP.

"If 100 countries say they want Assad to go that's real progress," he said. "The Syrian regime feels protected by the Russians and the status quo."

Makhous played down expectations of such an outcome given fractured opposition talks in Cairo.

"Let's not fantasise about a unified position. The challenge is to agree on a transition plan," he said.