The six Gulf states recognised a newly formed opposition bloc as the Syrian people's legitimate representative on Monday, as border violence stoked fears of a spillover of the 20-month conflict.
The Gulf Cooperation Council (GCC) move came exactly a year after the Arab League suspended Syria's membership, and as the National Coalition met Arab foreign ministers in Cairo buoyed by the hard-won unity deal.
The League late on Monday called the coalition the opposition's "legitimate representative and main interlocutor with the Arab League," and urged more opposition groups to join it.
"It is the legitimate representative of the Syrian opposition," Qatari Prime Minister Sheikh Hamad bin Jassem al-Thani, who heads the League group for Syria, told reporters.
His comments came after fighting flared on Syria's border with Turkey Monday as Israel fired across the ceasefire line on the Golan Heights for a second day, scoring direct hits on the source of a mortar round that struck the Israeli-occupied part of the territory.
Under Sunday's deal, the opposition agreed to establish a new supreme military council to take overall command of rebel groups on the ground and address US concerns that weapons have been reaching jihadist groups threatening to hijack the uprising.
The GCC members -- Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates -- were the first to recognise the opposition coalition.
"The states of the council announce recognising the National Coalition... as the legitimate representative of the brotherly Syrian people," GCC chief Abdullatif al-Zayani said.
He said the oil-rich bloc would support the coalition in the hope that "this will be a step towards a quick political transfer of power."
The premier of Qatar, whose government hosted marathon four-day talks that culminated in Sunday's unity deal, said earlier he would seek "full recognition" of the coalition.
His minister of state for foreign affairs, Khaled al-Attiya, said recognition would remove obstacles to the opposition securing arms for rebels.
The National Coalition's newly installed leader, Ahmed Moaz al-Khatib, told Al-Jazeera television it already had promises of weapons, but did not say from whom.
-- 'At least 107 killed Monday' --
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Washington swiftly declared its backing for the coalition.
"We look forward to supporting the National Coalition as it charts a course toward the end of Assad's bloody rule and the start of the peaceful, just, democratic future that all the people of Syria deserve," State Department spokesman Mark Toner said.
Traditional Damascus ally Moscow gave a cooler response, saying "such alliances must act based on a platform of peaceful regulation of the conflict by Syrians themselves, without interference."
It urged the opposition to drop its refusal to negotiate with the regime.
The head of Lebanon's Shiite militia Hezbollah, another Damascus ally, criticised the National Coalition's rejection of any political solution. It was "dangerous" and would lead to "more destruction," Hassan Nasrallah said.
Fanning international concerns about the potential of a spillover, the Israeli army targeted the source of a mortar round fired into the part of the Golan Heights it occupies and reported scoring "direct hits".
On Sunday, an Israeli warning shot -- its first across the UN-monitored ceasefire line since the 1973 Middle East war -- left UN chief Ban Ki-moon "deeply concerned by the potential for escalation," his spokesman said.
Air strikes and shelling of rebel positions in the town of Ras al-Ain on the Turkish border on Monday killed at least 15 people, said the Syrian Observatory for Human Rights.
One bomb exploded less than 150 metres (yards) from the border, with the blast blowing out windows in Ceylanpinar, where there were also injuries, the Turkish town's mayor told Anatolia news agency.
Near the town, helicopters strafed rebels who for the past three days have besieged an army post, said the Observatory.
At the far eastern end of Syria's border with Turkey, Kurdish militia took Malikiya village after troops loyal to President Bashar al-Assad withdrew, and on the Jordanian frontier, shelling and clashes left two Syrian border guards dead.
Warplanes also bombed rebel positions on the strategic highway between Damascus and second city Aleppo, while clashes on the southern outskirts of the capital near a Palestinian refugee camp killed seven civilians.
At least 107 people were killed nationwide on Monday, said the Observatory, which has given an overall death toll of more than 37,000 since March 2011.