* Western powers reluctant to supply weapons
* Rebels close in on Damascus, explosion rocks capital
* Obama announces U.S. will recognise Syrian opposition coalition
By Samia Nakhoul and Khaled Yacoub Oweis
MARRAKECH, Morocco, Dec 12 (Reuters) - Western and Arab nations sympathetic to Syria's uprising against President Bashar al-Assad gave full political recognition on Wednesday to the opposition, reflecting a hardening consensus that the 20-month-old uprising might be nearing a tipping point.
Meeting in the Moroccan city of Marrakech as rebels battled Assad's troops on the outskirts of his Damascus power base - and shortly before the capital was rocked by a huge explosion - the "Friends of Syria" group called on Assad to step aside.
But tensions between the United States and the Syrian opposition coalition surfaced at the same meeting when its leader criticised Washington's designation of the Islamist Jabhat al-Nusra rebel brigade as a terrorist organisation.
Hours earlier, U.S. President Barack Obama announced Washington would recognise the coalition of opposition groups, led by Sunni Muslim cleric Mouaz Alkhatib, as Syria's legitimate representative, joining France, Britain, Turkey and Gulf states.
"Participants acknowledge the National Coalition as the legitimate representative of the Syrian people and the umbrella organisation under which the Syrian opposition are gathering," said the Marrakech declaration after a meeting attended by 130 ministers and officials.
The gathering brought together Western and Arab nations opposed to Assad, whose family has ruled Syria for 42 years. But it excluded Russia, China and Iran, which have backed Assad or blocked efforts to tighten international pressure on him.
"Bashar al-Assad has lost legitimacy and should stand aside to allow a sustainable political transition," said the text.
Qatar reinforced that message after the meeting, urging Assad to take the "brave decision" to step down, while Britain called on him to end a war it said he could not win.
Referring to Western reports suggesting Assad might use chemical and biological weapons, the text said "any use of chemical weapons in Syria would be abhorrent and that this would draw a serious response from the international community".
Participants announced the creation of a relief fund "to support the Syrian people", calling on states and organisations to make contributions to the fund.
The resolution called on the U.N. Security Council, which has been paralysed by major powers' splits over Syria, to come up with "a meaningful and robust response" to the crisis and urged nations that support Assad to reconsider their positions.
NO PLEDGE OF ARMS
Although the text made no explicit commitment to arm the rebels, a diplomat following the talks said participants agreed on "the legitimate need of the Syrian people to defend itself against the violent and brutal regime of Bashar al-Assad".
Another diplomat said Western powers did not rule out supplying arms to rebel units in the future, but would want assurances about where the weapons would flow - pointing to several atrocities committed by rebel fighters and the presence of radical Islamists in their ranks.
"No option is ruled out. But there are big issues about the legality of intervening in a civil war. Any support to any group depends on the command control and the discipline on the ground," a Western diplomat at the Marrakech meeting said.
France said at the talks it was not ready to supply arms.
"For now we have decided not to move on this," French Foreign Minister Laurent Fabius told reporters in Morocco. "We shall see in the coming months."
Western officials are due to meet commanders of a newly formed rebel military command in Turkey next week.
Syria's state news agency SANA said Obama's recognition of the political opposition, which coincided with Washington's designation of the radical Islamist Jabhat al-Nusra group, part of the rebel force against Assad, as a terrorist organisation "proves American hypocrisy".
Russia also criticised the U.S. recognition, saying it ran counter to an agreement to seek political transition. Foreign Minister Sergei Lavrov said that it appeared the United States was betting on "armed victory" of Assad's opponents.
Coalition leader Alkhatib urged the United States to reconsider its decision to designate the militant Islamist Jabhat al-Nusra as a terrorist group.
"The decision to consider a party that is fighting the regime as a terrorist party needs to be reviewed," he said.
"We might disagree with some parties and their ideas and their political and ideological vision. But we affirm that all the guns of the rebels are aimed at overthrowing the tyrannical criminal regime."
Alkhatib called on Syria's Alawite minority to launch a campaign of civil disobedience against Assad, an Alawite facing a mainly Sunni Muslim uprising against his rule.
Fighting is moving closer to Assad's residence in the centre of Damascus, and early on Wednesday government forces fired artillery and rockets at southwestern suburbs of the capital adjacent to the Mezzeh military airport, activists said.
State television reported an explosion at the gate of the Interior Ministry in Damascus. The ministry is in Kafar Souseh, an area contested by rebels and forces loyal to Assad. A resident reported sirens and shooting after a "huge explosion."
SANA said on Wednesday that "terrorists" detonated two bombs in the Damascus district of Jaramana, killing one person and wounding five, and another two bombs behind the Justice Ministry in Damascus, wounding one person.
In central Syria, an attack on a village killed or injured as many as 200 members of Assad's Alawite minority sect, activists said, but it was unclear who was behind the assault.
The mainly Sunni Muslim rebels have scored a string of victories against Assad's forces, many of them from his Alawite religious minority. There is little evidence that the government is regaining control, residents say.
"We send a direct message to the Alawite brethren. The Syria revolution is extending its hand to you, so extend your hand back and start civil disobedience against the regime because it repressed you like it repressed us," said Alkhatib.
Alkhatib, elected last month as leader of the National Coalition for Opposition Forces and the Syrian Revolution, urged Assad's allies Iran and Hezbollah to withdraw their support.
The Syrian army is using warplanes and heavy artillery to try to halt further advances by rebels, many of them die-hard Islamists. Opposition leaders say they need heavy weapons to sustain the momentum and change the military equation in a conflict that has killed 40,000 people since March 2011.
The rebels now hold a near continuous arc of territory from the east to the southwest of the capital. With conditions deteriorating, Damascus residents face power and food crises.
Assad's political and armed opponents, dogged by splits and rivalries throughout their battle to end his rule, have established a more unified political opposition and military command, hoping to win international support.
Qatar and Saudi Arabia are already arming and financing the Syrian Muslim Brotherhood and other militant groups, while Iran is bankrolling Assad.
Brotherhood deputy leader Farouq Tayfour, speaking in Marrakech, said recent military gains by the rebels meant that Assad's end was rapidly approaching.
"The rebels are now much closer to the palace. Bashar is under siege. His end will be like Gaddafi's end. Didn't Bashar say, 'I was born in Syria and will die in Syria'? This is what Gaddafi said as well, and that's it."