France recognised the newly formed opposition National Coalition on Tuesday as the sole representative of the Syrian people, the first Western country to do so, and said the question of arming them must now be reviewed.
Meanwhile, fighting raged on in the country, with at least 115 people killed, the Syrian Observatory for Human Rights said.
"I announce that France recognises the Syrian National Coalition as the sole representative of the Syrian people and thus as the future provisional government of a democratic Syria, allowing an end to the Bashar al-Assad regime," President Francois Hollande told a press conference in Paris.
Earlier, coalition chief Ahmed Moaz al-Khatib called on world powers to arm Assad's foes, saying they desperately needed "specialised weapons" in order to "cut short the suffering of the Syrians and their bloodshed."
Hollande said the question of arming the rebels, hitherto opposed by France, would have to be reviewed.
"This question will have to be necessarily reviewed not only in France but in all countries which will recognise this government," he said.
For its part, the United States said the coalition was "a legitimate representative" of the Syrian people, but stopped short of recognising it as the sole representative.
State Department deputy spokesman Mark Toner said "we do think this is a legitimate representative of the Syrian people, that it does reflect the Syrian people... that diverse group of Syrian people."
Britain has said it wants to see more evidence that the grouping has strong support inside Syria before formally recognising it.
The French move came 24 hours after the coalition was recognised by the six member states of the Gulf Cooperation Council: Saudi Arabia, Bahrain, the United Arab Emirates, Oman, Qatar and Kuwait.
The diverse forces involved in the coalition agreed on Sunday to unify their fighting forces under a supreme military council and set up a national judicial commission for rebel-held areas in Syria.
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They plan to form a provisional government once the coalition has been widely recognised internationally.
EU foreign ministers meeting in Cairo on Tuesday welcomed the bloc and urged it to bring in more regime dissenters, with Khatib responding that "it is the strongest coalition and represents Syria internally."
"Many groups have joined. Some have reservations, and we are in touch with everyone. The vast majority has joined," he said in a telephone interview in Cairo.
The 22-member Arab League has stopped short of granting the bloc full recognition, stating only that it saw the alliance as "the legitimate representative of the Syrian opposition".
On the ground, fierce battles and army shelling in Damascus province killed more than 40 people, most of them civilians, while warplanes again bombed Ras al-Ain, a strategic town on the Turkish border, a watchdog said.
The fighting in the Eastern Ghuta area east of Damascus came after rebels launched an attack on public buildings in the area, the Britain-based Syrian Observatory for Human Rights said.
The army used tanks to shell several towns east of Damascus, including Harasta, Zabadani and Irbin, killing at least seven civilians including an unknown number of women and children, the Observatory said.
The Observatory also reported fresh air raids on Ras al-Ain, in northeastern Syria on the border with Turkey, and said 1,000 government troops had been sent to the town.
The air strikes have sent a new wave of civilians pouring into Turkey, adding to the 9,000 refugees who fled late last week when rebels overran the town, an AFP photographer said.
In other violence, the army shelled rebel positions in the southern province of Daraa, in the central province of Homs, in Idlib in the northwest and in the northern city of Aleppo, said the Observatory.
The Britain-based Observatory, which relies for its information on a network of activists, lawyers and medics, has given an overall death toll of more than 37,000 since the revolt broke out in March 2011.
Meanwhile, the Syrian Arab Red Crescent estimates that at least 2.5 million people have been internally displaced by the conflict, the United Nations said on Tuesday.