The United States is set to call for tough new UN sanctions against President Bashar al-Assad and his inner circle, as over 100 Western and Arab nations meet on Friday in Paris for 'Friends of Syria' talks.

But despite the more than 16,500 people estimated killed in the 16-month uprising against Assad's regime, key Syria allies Russia and China -- which both hold UN veto rights -- are not attending despite reports of changing attitudes in Moscow.

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 and Arab nations Saudi Arabia and Qatar led a group of more than 60 members, including most EU states and many Arab League nations.

The United States will lead calls at the Paris talks for a tough new UN sanctions regime to be imposed on Assad and his inner circle, Washington officials said Thursday.

Speaking as US Secretary of State Hillary Clinton flew to Paris for the Friends of Syria meeting, one 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," the official said aboard Clinton's plane and asking to remain anonymous, 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 last year. 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.

The talks will include a condemnation of the repression and an announcement of 'concrete measures' to put pressure on the regime and to support the people and the opposition, according to a Western diplomatic source.

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 wanted the Paris meeting to call for the UN Security Council to force Damascus to apply international envoy Kofi Annan's peace plan.

"We demand the Syrian file be sent to the Security council with a Chapter VII resolution to force the regime to implement the Annan plan," said Makhous.

"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.