Taking al Shaddadeh in Hasakah from President Bashar al Assad forces brings rebels closer to the provincial capital Hasakah, 30 miles to the north.
Coinciding with the report by the Syrian Observatory for Human Rights on the fighting was a video posted on YouTube showing fighters from the al Qaeda-linked Nusra Front celebrating and chanting "Allahu Akbar (God is Greatest), Nusra Nusra".
A spokesman for the eastern command of the rebel Free Syrian Army, said rebels including Nusra fighters controlled al Shaddadeh after over-running state security and military intelligence compounds.
Hasakah, an ethnically mixed province of Arabs and Kurds, accounts for most of Syria's oil output, which is thought to have fallen by one-third to no more than 100,000 barrels per day since an uprising against Mr Assad's rule erupted in March 2011.
A rebel spokesman said an army garrison at a gas field, al Ghabsa, had fled as rebels advanced.