Is North Korea trying to restart a nuclear reactor?

The US-Korea Institute said on its 30 North website that new satellite imagery showed signs of activity at the Yongbyon reactor, including steam and indications that snow had melted on the reactor roof.

But the monitoring project at Johns Hopkins University in Washington, said it was too early to reach a "definitive conclusion."

It said: "One possibility is that the North Koreans are in the early stages of an effort to restart the reactor after an almost five-month hiatus in operations. 

"However, since the facility has been recently observed over a period of only a few weeks, it remains too soon to reach a definitive conclusion on this and also on whether that effort is moving forward or encountering problems."

Yongbyon nuclear complex was shut down in 2007. 

But the country revived the five-megawatt research reactor in 2013 saying it was seeking a deterrent capacity - a move condemned by the UN nuclear watchdog, the International Atomic Energy Agency.

Observations of commercial satellite images from December 24, 2014 and January 11, 2015, showed possible signs of a restart because of hot water drainage from a pipe at the turbine building, which indicates steam from the reactor. 

The think tank claimed the absence of the foam could also be related to the installation of new piping.

Activity at the reactor was last detected in November - the same month North Korea threatened to launch a fourth nuclear test in spite of a United Nation's move to investigate the country's human rights violations. 

North Korea is under an array of international sanctions for repeated nuclear bomb and ballistic missile tests.

The country has previously conducted nuclear tests in 2006, 2009 and 2013. 

Earlier this month it offered to suspend nuclear tests if the United States called off annual military drills with Seoul, South Korea. 

But Washington rejected the proposal as a veiled threat.