* Syria's Kurds see civil war as chance for autonomy
* Have made military gains against non-Kurdish rebels
* Declare "transitional, pluralistic" administration
ARBIL, Iraq, Nov 12 (Reuters) - Ethnic Kurds declared an interim administration in northeastern Syria on Tuesday, further solidifying their geographic and political presence after driving out Islamist rebels.
Long oppressed under Syrian President Bashar al-Assad and his father before him, Kurds view the civil war as an opportunity to gain more autonomy - like their ethnic kin in neighbouring Iraq.
Control over Syria's northeast, where Kurds predominate, had in recent months swung back and forth between them and mainly Arab Islamist rebels, who strongly oppose what they suspect are Kurdish plans to seccede.
But a Kurdish militia prevailed earlier this month, and at a meeting held in the Syrian city of Qamishlo on Tuesday, a committee of Kurdish and other groups said it was now time to set up an administrative body to run the region.
"In light of the current circumstances which Syria is going through, and in order to fill an administrative vacuum ... we see is as utmost necessity to reach a transitional, pluralistic, democratic administration," said a statement sent to Reuters.
The statement said they were committed to the unity of Syria and asked world powers and neighbouring countries to back the new administration, which they said had won the support of different political groups and minorities in the area.
Kurdish empowerment in Syria has caused concerns elsewhere, not least in neighbouring Turkey, which has fought a three decade war against the Kurdistan Workers' Party (PKK).
The dominant force on the ground in Syria's Kurdish areas is the Democratic Union Party (PYD), which has a well-trained militia and is aligned with the PKK, although they maintain they are distinct.
The PYD's growing clout has also dismayed some fellow Kurds, who accuse it of being in league with Assad and seeking to replace his authoritarian one-party rule with its own. PYD representative Mohammed Reso said some Syrian Kurdish parties had refused to sign up to the plan.
Numbering more than 25 million, non-Arab Kurds are often described as the world's largest ethnic group without a state. Territories where they predominate, which they call Kurdistan, span parts of Turkey and Iran as well as Syria and Iraq. (Writing by Isabel Coles; Editing by Robin Pomeroy)