Countries opposed to President Bashar al-Assad's regime meet in Morocco on Wednesday for talks on Syria's 21-month conflict after the US gave official backing to a new opposition bloc.

The Friends of Syria group meeting of Arab and Western states coincides with battlefield gains by Islamist rebels suspected of Al-Qaeda links, and a rapidly deteriorating refugee situation as winter sets in.

Headlining the talks, which are to begin in Marrakesh at 0930 GMT, are two key issues, namely the political transition after Assad's eventual fall and mobilising humanitarian aid.

With the total death toll from the civil war now topping 42,000, according to monitors, the UN refugee agency said 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 National Coalition, US President Barack Obama 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 said on the eve of the Morocco meeting.

Russia, the Assad regime's most powerful ally, expressed surprise at the move, with Foreign Minister Sergei Lavrov saying "the United States has decided to place all its bets on an armed victory of the National Coalition".

Under pressure to unite, 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.

But jihadist rebels in Aleppo, a key front line in northern Syria, rejected the coalition, saying they want an Islamic state.

Among them was the Al-Nusra Front, which the United States blacklisted on Tuesday as a "terrorist" organisation.

Obama's recognition 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 coalition members had expressed confidence the opposition group would consolidate its international backing as a viable alternative to 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," said German Foreign Minister Guido Westerwelle.

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 was the US decision to blacklist the Al-Nusra Front, which it accuses of hijacking the Syrian uprising.

Obama on Tuesday repeated State Department claims that Al-Nusra, while portraying itself as part of the legitimate 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 said.

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

Germany said late Tuesday it would provide another 22 million euros ($29 million) in humanitarian aid for victims and refugees of the conflict, bringing its total contribution to 90 million euros.

Westerwelle said the funds would go in particular to the Red Cross, the World Food Program and the UN High Commissioner for Refugees.