The Friends of Syria nations opposed to President Bashar al-Assad meet in Morocco on Wednesday to look afresh at how to resolve the deepening crisis and support the new opposition group after it won official US backing.

The meeting in Marrakesh coincides with a rapidly deteriorating refugee situation as winter sets in, and gains in the battlefield by a key rebel group suspected of links to Al-Qaeda.

Headlining the agenda for Arab and Western states are two key issues, namely the political transition after Assad's eventual fall and mobilising humanitarian aid.

With the total death toll from the conflict now topping 42,000, according to a monitoring group, the UN refugee agency said on Tuesday that the number of Syrian refugees who had fled to neighbouring countries and north Africa had now passed half a million.

But in a major boost for the newly-formed opposition coalition, and the most significant US intervention yet in the brutal civil war, President Barack Obama on Tuesday endorsed the group as "the legitimate representative" of the Syrian people.

"We have made a decision that the Syrian opposition coalition is now inclusive enough, is reflective and representative enough of the Syrian population, that we consider them the legitimate representative of the Syrian people," Obama told ABC News in an interview, on the eve of the Morocco meeting.

Under international pressure to unite, after struggling for months to do so, the Syrian opposition agreed in Doha on November 11 to establish the national coalition and group the various rebel forces under a supreme military council.

Obama's announcement follows a similar move by the European Union on Monday, after EU foreign ministers met the coalition's leader, Ahmed Moaz al-Khatib.

Earlier, Western diplomats and members of the newly-formed rebel National Coalition had expressed confidence that the opposition group would consolidate its international backing as a viable alternative to President Assad's beleaguered regime.

"It is our goal that, after the European Union, now the 130 states in the Friends of Syria group will send a message of recognition to the national coalition of the Syrian opposition," German Foreign Minister Guido Westerwelle said on Tuesday, before flying to Morocco.

Al-Qaeda concerns

Despite the growing recognition, several EU member states have expressed reservations about the group, in terms of how representative it is and its democratic commitment.

Reflecting another potential threat to the coalition's credibility -- and to future financial and military support -- Washington on Tuesday blacklisted Al-Qaeda-linked rebel group Al-Nusra Front that it accuses of hijacking the uprising in Syria.

Al-Nusra fighters, many of them jihadist volunteers from around the Islamic world, were instrumental in the fall of the army's massive Sheikh Suleiman base in northern Syria on Monday after a months-long siege.

Obama on Tuesday repeated State Department claims that Al-Nusra, while portraying itself as part of the legitimate Syrian opposition, was in fact a front for Al-Qaeda in Iraq.

A spokesman for the national coalition insisted, however, that there were "inaccuracies" around Al-Nusra.

Yaser Tabbara said the extremist group could be divided into two factions -- one that supported the regime and committed acts of terror, and the other that did not -- and urged dialogue with the latter.

"They're not going to be a stumbling block in the recognition of the coalition," he told AFP.

Tabbara also underlined hopes that the Marrakesh meeting would help alleviate Syria's mounting humanitarian crisis and support the financial needs of the liberated areas, in terms of salaries and services, which the group estimates at nearly $500 million per month.

"The amount of aid that we need and that we expect is enormous, and we really hope that our friends and allies deliver," he said.

Syria witnessed more carnage on Tuesday, with bomb attacks in a village in the central Hama province killing or wounding at least 125 civilians from President Assad's minority Allawite sect, the Syrian Observatory for Human Rights said.