The US-Korea Institute at Johns Hopkins School of Advanced International Studies said a satellite image from August 31 shows white steam rising from a building near the hall that houses the plutonium production reactor's steam turbines and electric generators.
The International Atomic Energy Agency (IAEA), the UN nuclear watchdog, said it did not yet have a "clear understanding" of the situation, while the US special envoy for North Korea said if the restart was confirmed, it would be a very serious development. Glyn Davies said: "It would violate a series of UN Security Council resolutions."
A US official said he believed the North Koreans had restarted the reactor, saying the amount of steam suggested that it was being tested.
The official said he did not think the North may have done so to force major powers to resume nuclear talks with it in the hopes of extracting concessions, but rather to demonstrate that it would not abandon its nuclear programmes.
A spokesman for the US State Department repeated Washington's long-standing call for North Korea to comply with a 2005 aid-for-disarmament agreement signed by North Korea, its neighbours and the US.
North Korea has conducted three nuclear tests since 2006 and James Acton, an analyst with a Washington think tank, said: "Restarting the reactor is another slap in the face to the international community, indicating North Korea has no intention whatsoever of abandoning its nuclear weapons."