Deadly fighting is raging near Damascus and in Syria's northeast, as the Arab League urges more groups to join a newly formed opposition bloc that won swift recognition from the Gulf states.
The Gulf Cooperation Council (GCC) said its six members recognised the National Coalition as "the Syrian people's legitimate representative", and the Arab League also gave its backing, although it stopped short of granting it full recognition.
The GCC members - Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates - on Monday became the first to recognise the umbrella group.
And foreign ministers of the 22-member Arab League said at talks in Cairo that they recognised the coalition as "the legitimate representative of the Syrian opposition".
They called on "the rest of the opposition to join" the bloc, and urged "regional and international groups to recognise (it) as a legitimate representative of the aspirations of the Syrian people".
The hard-won coalition deal reached on Sunday in Doha, Qatar, calls for the opposition to create a supreme military council to take overall command of rebels fighting to oust President Bashar al-Assad's regime.
The bloc's newly appointed leader, moderate Muslim cleric Ahmed Moaz al-Khatib, said the coalition already had promises of weapons, without specifying from whom.
The United States swiftly declared its backing for the National Coalition following Sunday's deal that brought together a broad spectrum of regime opponents.
"We look forward to supporting the National Coalition as it charts a course towards the end of Assad's bloody rule and the start of the peaceful, just, democratic future that all the people of Syria deserve," said the State Department.
Traditional Damascus ally Moscow gave a cooler response, urging the opposition to drop its refusal to negotiate with the Assad regime.
France said on Tuesday it would support the new opposition bloc, but also fell short of granting it full recognition.
"Now they are united, it's very important ... France will support them," Foreign Minister Laurent Fabius said at a meeting of European and Arab foreign ministers at the League's Cairo headquarters.
EU Foreign policy chief Catherine Ashton, who addressed the opening session at the League, welcomed the Doha agreement but warned of a spillover of the conflict.
"I want to welcome the work done in Doha to build and bring together the opposition, to be inclusive of the people in Syria to be determined in the offer they make to the people," said Ashton.
"But the tragedy of Syria is a tragedy that affects not just that country but the whole region," she added.
Outgunned rebel fighters have been battling to secure a buffer zone along the border with Turkey for the past few months.
And regime warplanes carried out a new wave of bombing raids on Tuesday on the strategic town of Ras al-Ain, on the northeastern border, a day after deadly air strikes and shelling, said the Syrian Observatory for Human Rights.
The Local Coordination Committees activist network said dozens of military vehicles were headed towards Ras al-Ain, while the Observatory reported heavy shelling.
Elsewhere, fierce clashes rocked in the east Damascus suburb of Ghuta and at Daraya to the south after rebel attacks on public buildings and a military checkpoint in the two areas, said the Observatory.
The air strikes in the northwest have sent a new wave of civilians pouring across the frontier to the Turkish side, adding to the 9000 refugees who fled late last week when rebels overran Ras al-Ain.
Violence on Syria's borders with Iraq, Jordan, Lebanon and Turkey, as well as across the UN-monitored ceasefire line that splits the Israeli-occupied Golan Heights, has stoked fears of a spillover of the 20-month conflict.
Israel fired across the ceasefire line for a second day on Monday, scoring direct hits on the source of a mortar round that struck the Israeli-occupied part of the territory.
In other violence on Tuesday, the army shelled rebel positions in the southern province of Daraa, in the central region of Homs, in Idlib in the northwest and in the northern city of Aleppo, said the Observatory.
At least 151 people were killed across Syria on Monday, including 61 civilians, said the Britain-based Observatory.
The watchdog - which relies on a network of activists, lawyers and medics for its information - has given an overall death toll of more than 37,000 since the revolt broke out in March 2011.
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