Nearly 19,000 Syrians, many from the capital, have fled from violence at home across the border into Lebanon in just one day, a Lebanese security source said.
"More than 18,600 Syrians have arrived since yesterday from Syria through the eastern Masnaa crossing, among them many people from Damascus and its province,'' the source said.
Previously, the United Nations High Commmissioner for Refugees said there were 26,900 Syrians refugees registered in Lebanon, although activists have put the total number much higher.
Meanwhile Syrian rebels took control of all border crossings between Iraq and Syria overnight NZ time, Baghdad's deputy interior minister told AFP.
"All the border points between Iraq and Syria are under the control of the Free Syrian Army," Adnan al-Assadi told AFP by telephone.
"The border points and all the outposts are under the control of the Free Syrian Army.
"The Syrian army are focusing on Damascus."
Assadi also said there was fighting under way between the Syrian army and rebels across the border from the northern Iraqi region of Sinjar.
The top official said Iraqi border guards had witnessed the Free Syrian Army take control of a border outpost, detain a Syrian army lieutenant colonel, and then cut off his arms and legs.
"Then they executed 22 Syrian soldiers in front of the eyes of Iraqi soldiers."
The account of the killings could not be independently verified.
Assadi added: "If this situation continues, we are going to close the entire border with Syria."
Earlier, an Iraqi border police lieutenant colonel told AFP that clashes began at noon at the Albu Kamal crossing in Anbar province."
"And then in the evening, we saw the Syrian flag being brought down and the flag of the Free Syrian Army replaced it," he told AFP, speaking on condition of anonymity.
"We saw people wearing dishdashas (traditional Arab robes) and holding guns and they were walking inside the border post. We closed the border and we sent more patrols and more checkpoints along all the border with Syria."
Government spokesman Ali Dabbagh meanwhile told Iraqiya state television that four planeloads of Iraqis had been flown from Syria back to Iraq on Thursday.
Also overnight, officials in western Anbar province, which borders Syria, said thousands of Iraqi families crossed from Syria back into their home country after Baghdad appealed for them to return to avoid escalating violence in Iraq's western neighbour.
Anbar provincial council chief Saadun Obeid Shaalan said provincial authorities had provided Iraqis crossing the border with "humanitarian aid and 60 buses have entered Iraq, carrying Iraqis emigrating from Syria."