By Jonathon Burch and Rania El Gamal
ANKARA/DOHA, Nov 9 (Reuters) - Thousands of Syrians fled their country on Friday in one of the biggest refugee exoduses of the 20 month war after rebels seized a border town.
Syria's fractious opposition was meeting in Qatar, under increasing pressure from the United States and Qatar to unite and form a credible body capable of ruling the country effectively if President Bashar al-Assad falls.
The United Nations said 11,000 refugees had fled in 24 hours, most to Turkey. The exodus is testing the patience of Ankara, the most militarily capable of Syria's neighbours and a strong opponent of Assad. Ankara has long said a full-blown refugee emergency would demand robust intervention.
Rebels overran the frontier town of Ras al-Ain late on Thursday, continuing a drive that has already seen them push Assad's troops from much of the north and seize several crossing points, a rebel commander and opposition sources said.
"The crossing is important because it opens another line to Turkey, where we can send the wounded and get supplies," said Khaled al-Walid, a commander in the Raqqa rebel division.
The Syrian Observatory for Human Rights, a UK-based group that compiles opposition activist reports, said at least 20 members of the Syrian security forces were killed when rebel fighters attacked a security headquarters in Ras al-Ain.
Thousands of residents poured out of the Arab and Kurd town, in the northeastern oil-producing province of Hasaka, 600 km (375 miles) from Damascus.
TURKEY HITS OUT
Turkey's Prime Minister Tayyip Erdogan hit out at world powers on the U.N. Security Council over their inaction.
"It is very strange. There are currently atrocities being committed in Syria and these atrocities are being directed by a state leader," he said. "How far will this go? When will the permanent members of the Security Council take responsibility?"
Turkey has responded in kind to mortar shells hitting its soil from fighting in Syria and is discussing with NATO allies whether to deploy Patriot air defence missiles on the border.
The Turkish state-run Anatolian news agency reported that 26 Syrian military officers had also arrived in Turkey with their families overnight in what it called the biggest mass desertion of senior soldiers from Assad's forces in months.
The latest flight of refugees raised the total recorded by the United Nations to 408,000. The International Committee of the Red Cross said on Thursday it could not keep up with the increasingly dire needs of civilians.
Advancing rebels managed to fire mortars at the presidential palace in Damascus this week. Residents in the capital said security was being beefed up there. Assad told Russia Today television on Thursday he would "live and die in Syria", echoing words of other Arab leaders before they lost power last year.
As rebels advance, an opposition Syrian National Council made up mainly of exiles has been seen as ineffective and out of touch with groups fighting on the ground. Western countries fear that means Islamist militants could seize power if Assad falls.
Qatar, which has bankrolled Assad's enemies and played a leading role in Arab diplomacy against him, hosted an opposition meeting in its capital Doha, with senior U.S. diplomats hovering on the sidelines and leaning hard on the opposition to unite.
A source inside meetings that lasted into the early morning hours said SNC members who had resisted a deal were now bowing to pressure for a new body that would give more voice to groups operating on the ground in Syria itself.
"We will not leave today without an agreement," the source said. "The body will be the sole legitimate representative of the Syrian people. Once they get international recognition, there will be a fund for military support."
An outline agreement could see the SNC and other opposition figures agree on a 60-member political assembly, mirroring the Transitional National Council in Libya, which united opposition to Muammar Gaddafi last year and took power when he fell.
"They will create a 'temporary government', which could take control of embassies around the world and take Syria's seat at the U.N., because the regime would have lost its legitimacy," said the source.
The SNC, which has previously been the main opposition group on the international stage, may have around a third of the seats in the new body but would otherwise lose much of its influence. It was not yet clear whether the meeting in Doha would name members or broach the thorny issue of the new body's leadership.
Qatar's prime minister told delegates on Thursday to "get a move on" in a closed meeting in a Doha five-star hotel.
"The Qataris are not to going to let them leave here in failure after all this investment," said a diplomatic source on the sidelines of the meetings. However, he sounded a note of caution: "Yes there will be an agreement, but is it sustainable? Is it well thought through and well prepared?"
U.S. Secretary of State Hillary Clinton last week called for overhauling the opposition, saying more representation was needed for those "on the frontlines and dying". The SNC is due on Friday to complete elections to its executive council and choose a new leader, before continuing talks with other groups.
Senior SNC member Burhan Ghalioun said the atmosphere at the talks was positive. "We all agree that we don't want to walk away from this meeting in failure."
In the last three months, the mainly Sunni Muslim Arab rebels have captured outposts on the Turkish border, moving towards the northeastern heartland of Syria's one million Kurds, many of whom have tried to stay clear of violence.
The Kurdish Council, a coalition of Kurdish parties opposed to Assad, called on rebels to pull their fighters out of Ras al-Ain, saying the clashes and fear of Syrian bombardment had prompted most of its 50,000 residents to flee.