"Since late August 2013, the agency has observed, through analysis of satellite imagery, steam discharges and the outflow of cooling water" at the reactor at Yongbyon, the International Atomic Energy Agency said.
It added that such activity is "consistent with the reactor's operation.
However, since the agency has had no access to the five megawatt reactor since April 2009, it cannot confirm the operational status".
|North Korea’s Yongbyon nuclear complex (Yonhap)|
The reactor is capable of giving North Korea, which has carried out three nuclear tests, six kilos (13 pounds) of plutonium a year -- enough for one nuclear bomb, experts say.
It shut the reactor down in 2007 under an aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013, and previous satellite images suggested renewed activities there.
The IAEA's comments, in a new annual report on North Korea, chime with an assessment by the US think-tank the Institute for Science and International Security (ISIS) last month, also using satellite images.
ISIS said however that "without more data, such as regular steam production, it is hard to determine the operational status of the reactor and thus to estimate the amount of plutonium produced".
The UN watchdog also noted "further renovations" at another new facility at Yongbyon that North Korea is thought to be building for enriching uranium, but that it "cannot confirm the purpose of these activities".
The North says the purpose is to produce low-enriched uranium for a new reactor it is constructing, but experts suspect that the real goal is weapons-grade uranium, an alternative to plutonium for a nuclear bomb.
The IAEA said that as of June 2013, external work on the building housing that new reactor "appeared completed" but that since then "little further activity" was observed, with "no indications of the delivery or installation of major components".
It reiterated that North Korea's activities were a "matter of serious concern". IAEA inspectors have not had access to North Korea since being kicked out in 2009. (AFP)