Russia harshly criticised Washington's decision to recognise Syria's main opposition group as the country's sole legitimate representative today.
Foreign Minister Sergey Lavrov said it contradicted earlier international agreements aimed at the "commencement of an all-Syria dialogue" that would include all sides of the conflict.
US President Barack Obama said on Tuesday night that the newly formed Syrian Opposition Council "is now inclusive enough" to be considered as the country's legitimate representative and claimed that it was "a big step" in international efforts to end Syrian President Bashar Assad's embattled regime.
The fourth conference for the so-called Friends of the Syrian People opened today with more than 100 delegates from Europe and the Gulf countries meeting in Marrakech, Morocco.
The conference, as exected, fell in line with the US position.
The Syrian National Coalition, formed in November during a conference in Qatar, has been pleading for increased international support, including arms.
But the US is not expected to hand over military aid because of the al-Qaida linked groups operating openly in the rebel forces.
President Obama's announcement followed his administration's blacklisting of a militant Syrian rebel group with links to al-Qaida.
The US had supposedly been leading international efforts to prod the fractured Syrian opposition into coalescing around a leadership that would represent all of the country's factions and religions.
But the blacklisting of Syria's al-Nusra Front made it clear that "all" meant "all that the US approved of."
The pre-emptive blacklisting of the group just hours before the conference was due to start meant that authorities can now freeze any assets the group or its members have in US jurisdictions.
It also prohibits US citizens from giving the group any material support.
The State Department blamed the group for nearly 600 terrorist attacks in Syria since November 2011, including more than 40 suicide attacks.
"Through these attacks, al-Nusra has sought to portray itself as part of the legitimate Syrian opposition while it is, in fact, an attempt to hijack the struggles of the Syrian people for its own malign purposes," the department claimed.
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