Speaking as US Secretary of State Hillary Clinton flew to Paris for Friday's 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."
The aim would be to keep up momentum, building on talks in Geneva last weekend and the Cairo and Paris talks, with the UN Security Council set to take up the sanctions issue as soon as next week.
"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 underway and the focus will be in New York next week as soon as we finish in Paris basically."
Friday's Friends of Syria meeting will bring together more than 80 nations, as well as non-governmental organizations and representatives of the Syrian opposition.
The US officials hailed a blueprint drawn up earlier this week by the Syrian opposition in talks held under the auspices of the Arab League in Cairo, which set out a clear plan for transition to a post-Assad era in Syria.
It also drew up the basis for a future constitution and governing system for the Arab nation, which has been ruled for decades by the Assad family and the Baath party.
According to the document, as soon as Assad steps down a new caretaker government would be installed to start the transition process.
It would aim to bring together a "wide national conference" in Damascus to include all political powers and across all spectrums of society.
This conference would then set up a temporary legislative body to work on a new constitution and hold parliamentary elections within a year. Following that election, the new government would have to put the constitution to a referendum within six months.
"It's a very explicit, almost a bill of rights in terms of each group. They all get to have their rights protected and their place in Syrian society without any split in the country, in the society, in the territory," said the second US official.
The document also makes it clear that those people with "blood on their hands" would have no role in the next governments, although the US official said it would be up to the Syrian people to make that determination.
"It is not something that we would define or we would dictate," the official said.
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