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* Insurgents in eastern city said to include Islamist unit
* More than 700,000 Syrians have fled the country
* Heavy fighting between rebels and Kurdish militants
BEIRUT, Jan 29 (Reuters) - Rebels in Syria including al Qaeda-linked Islamist fighters captured a security agency in the eastern city of Deir al-Zor after days of heavy fighting, an activists' video and a pro-opposition monitoring group said on Tuesday.
A video posted online showed men armed with assault rifles cheering as they stood outside a building they said was the local intelligence agency branch.
Some of the fighters carried a black flag with the Islamic declaration of faith and the name of the al-Nusra Front unit, which has ties to al Qaeda in neighbouring Iraq.
The Britain-based Syrian Observatory for Human Rights said the fighters freed prisoners from the building.
"This was after violent clashes continuing for a number of days," the group said. It said it did not have information about casualties.
It was not clear when the building was captured and it is not possible for Reuters to verify reports inside Syria because of restrictions on independent media.
The online video also showed tanks, which appeared to be damaged, and a room containing weapons.
Fighting also took place in the northern town of Ras al-Ain on the border with Turkey, between rebels and Kurdish militants, the observatory said on Tuesday.
The insurgents have been battling fighters of the Kurdish People's Defence Units for about two weeks in the area, and scores of people have died in the violence.
Syria's 22-month-old revolt started off as a peaceful protest movement against more than four decades of rule by President Bashar al-Assad and his family, but rebels took up arms after a government crackdown.
The ensuing civil war has killed more than 60,000 people and has led to more than 700,000 fleeing the country, the United Nations says.
The rebels have taken territory from the government in the north and east, but have failed to capture major cities and are disorganised, fragmented and ill-equipped compared with government forces. (Reporting by Alexander Dziadosz and Ayat Basma; Editing by Pravin Char)