The Gulf Cooperation Council said its six members recognised the National Coalition as "the Syrian people's legitimate representative", and the Arab League also gave its backing.
The 22-member League, however, stopped short of granting the bloc full recognition, stating only that it saw the alliance as "the legitimate representative of the Syrian opposition".
It called on the rest of the opposition to join, and urged regional and international groups to recognise it as a legitimate representative of the Syrian people's aspirations.
EU foreign ministers meeting at the League's headquarters in Cairo took a similar stance, welcoming but declining to recognise the alliance while calling on it to bring in more regime opponents.
"It is a very important milestone and a very big step towards (recognition)," British Foreign Secretary William Hague said on the sidelines of the EU-Arab gathering.
The Europeans wanted to ensure the deal was implemented, and to see that the coalition is "as as possible of opposition groups and all communities in Syria," he said.
Rebel fighters in Syria dismissed the Gulf and Western support for the new alliance, expecting little to change on the ground unless they get cash and weapons.
"For one-and-a-half to two years, we've paid blood for our freedom. People have left their homes and die here every day and no one cares," said Abu Osama, a 40-year-old rebel manning a checkpoint close to the Syrian-Turkish border.
"If they do something we can see on the ground, that's better than sitting outside the country in hotels, eating and drinking while we're here dying every day," Abu Osama told AFP.
International rights groups said, however, that the opposition's new leadership must act to monitor and curb any violations of international law by the rebels.
"Syria's newly created opposition front should send a clear message to opposition fighters that they must adhere to the laws of war and human rights law, and that violators will be held accountable," said Human Rights Watch.
The hard-won coalition deal reached 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 coalition, but Damascus ally Moscow urged the opposition to drop its refusal to negotiate with the Assad regime.
EU foreign policy chief Catherine Ashton, speaking in Cairo, welcomed the unity deal but warned "the tragedy of Syria is a tragedy that affects not just that country but the whole region".
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 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 Daraya to the south after rebels attacked public buildings and a military checkpoint in the two areas, said the Observatory.
The air raids in the northwest have sent a new wave of civilians pouring across the Turkish frontier, adding to the 9,000 refugees who already 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 ceasefire line that splits the Israeli-occupied Golan Heights, has stoked fears of a spillover of the 20-month conflict.
Israel fired across the UN-monitored line for a second day Monday, scoring direct hits on the source of a mortar round that struck the Israeli-occupied part of the territory.
In other violence 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 29 people were killed across Syria on Tuesday, including eight civilians, it said.
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.