The National Intelligence Service told lawmakers from the National Assembly's Intelligence Committee the 5 megawatt reactor, which had been shut down in 2007, has been up and running since about August, Yonhap News Agency reported.
"The reactor is currently being operated to increase the North's nuclear capacity," The Korea Times quoted NIS head Nam Jae-joon as saying.
Nam said the North is also testing engines for long-range missiles, the Times said.
The newspaper noted the North had torn down the reactor's cooling tower in 2008 in return for shipments of food aid and to demonstrate its seriousness about suspending nuclear activities under an agreement reached at the six-party denuclearization talks.
But North Korea has resumed test firings of its long-range ballistic missiles and conducted a nuclear bomb test early this year over international objections. The North announced in April it would restart work at Yongbyon.
The Los Angeles Times reported scientists at 38 North, a website operated by the U.S.-Korea Institute, said last month satellite imagery from Sept. 19 showed "hot waste water being released into the Kuryong River from a recently installed drainpipe, part of a new secondary cooling system completed in summer 2013." The website said the heated outflow from the reactor wasn't visible in July.