Satellite images have shown a "high level of activity" at North Korea's key nuclear facility, a U.S. organization said, as leader Kim Jong Un has ordered an expansion of the country's production of weapons-grade nuclear materials.

The images taken in March suggest that completion of an experimental light water reactor at the nuclear complex in Yongbyon is just around the corner, Washington-based website 38 North, a group monitoring North Korea, said Saturday.

Water discharges have also been "detected that could be associated with testing" the reactor's cooling system, the organization said, adding the move indicates the device is "nearing a transition to operational status."

In late March, the state-run Korean Central News Agency reported that Kim said his nation "should continuously strive to strengthen its nuclear force steadily."

Pyongyang has launched a series of ballistic missiles recently, with fears mounting that the country may be preparing to carry out its seventh nuclear test, the first since September 2017.

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