The announcement, hailed as a watershed moment for the movement, will be made at the Friends of Syria summit in Marrakech attended by regional and western backers of the opposition.
The event is intended to cement claims by the Syrian National Coalition (SNC) to be an alternative to the government of the embattled president, Bashar al-Assad.
The US has signalled it will recognise the SNC as a "legitimate" representative of the Syrian people because it has moved to organise itself into a more inclusive and relevant body. Britain, France, Turkey and some Gulf states announced their endorsements last month.
Hillary Clinton, the US secretary of state, had been expected to announce the US move on Wednesday, but she has fallen ill with a stomach virus and the deputy secretary of state, Bill Burns, will attend the event instead.
Western legitimacy is seen by opposition leaders as an essential step to attract support among Syrians, many of whom, fatigued by 21 months of war, would prefer Assad to remain in power if it prevented a further descent into chaos.
The opposition movement plans to move aid into Syria through local activists and newly formed civil administrations. It is expected to try to bypass the Free Syria Army (FSA) networks, but will accept help from the main rebel group in securing safe corridors.
Only a trickle of aid has reached increasingly desperate communities inside Syria, where up to 2 million people are thought to have been internally displaced.
The United Nations announced on Tuesday that the number of registered refugees outside the country had topped 500,000 and was continuing to rise daily.
The crumbling of state authority is being felt in most parts of the country. The vacuum being created appears to have galvanised into action critics of the Assad regime who have so far remained largely on the sidelines.
Moves against the regime have so far centred on sanctions against key figures and the Syrian economy, parts of which are steadily grinding to a halt.
However, diplomatic moves have sharply intensified over the past week and prospects have risen of a military intervention, an unpopular option in Whitehall and the Pentagon.
The increased interest in the Syrian crisis in Washington, London and Paris is in part due to fears that cornered Syrian officials may authorise the use of chemical weapons. The government in Damascus has strongly denied it would use its stockpiles of sarin and mustard gas, claiming "terror groups" may instead do so.
The US officially proscribed a jihadist group fighting in Syria, Jabhat al-Nusra, as a terrorist organisation on Tuesday, alleging it is an alias for al-Qaida in Iraq (AQI) that has claimed responsibility for about 600 attacks, including 40 suicide bombings.
An official from Barack Obama's administration, speaking on condition of anonymity, said the transition to a post-Assad government was gathering speed and the US did not want extremists dictating the shape of the transition.
He said the al-Nusra Front for the People of the Levant rejected the mainstream Syrian opposition groups' vision of a tolerant society and free elections. "It is an extremist organisation that has to be isolated," the official said.
State department spokeswoman Victoria Nuland said: "Al-Nusra has sought to portray itself as part of the legitimate Syrian opposition while it is, in fact, an attempt by AQI to hijack the struggles of the Syrian people for its own malign purposes. AQI emir Abu Du'a is in control of AQI and al-Nusra."
Abu Shahid, a fighter with the Tawhid brigade, was among several FSA fighters who reacted angrily to the US decision. "Jabhat al-Nusra are not terrorists. They are working hard for us. They are fighting with us," he said. "Jabhat al-Nusra has been helping Syria. The Americans by contrast have done nothing. And now they say everyone with a beard is a terrorist."
Shahid said his unit mostly conducted its own operations but had co-operated with jihadist fighters in the battle for Aleppo. "The boys from Jabhat al-Nusra are brave. They give food and supplies to the people," he said.
Shahid expressed provisional backing for Syria's new opposition coalition: "We like the coalition for now. But when the regime is finished, we will see. If we don't like it, we will make a new government."
Haji Abu Mohamed, a Syrian living near Aleppo, responded more positively to the US clampdown on Jabhat al-Nusra. "We want normal Islam, and a normal Islamic country. We need democracy. That's our target," he said.
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