The Donga Ilbo newspaper reported earlier in the day that intelligence sources in Seoul and Washington had detected signs the five-megawatt reactor in Yongbyon had temporarily stopped operations late last month.

The suspension could be an indication that spent fuel rods are being reprocessed to extract plutonium for use in nuclear weapons, the report cited a government source as saying.

"South Korean and US intelligence authorities are closely monitoring related movements," defence ministry spokesman Jeon Ha-kyou told reporters when asked about the report at a daily briefing.

Located about 100 kilometres (60 miles) north of Pyongyang, Yongbyon is home to the country's first nuclear reactor and the only known source of plutonium for North Korea's banned weapons programme.

Pyongyang last week enshrined its status as a nuclear power in its constitution, with leader Kim Jong Un calling for more modern atomic weapons to counter perceived threats from the United States.

North Korea has conducted a record number of missile tests this year in the face of international sanctions, ignoring warnings from the United States, South Korea and their allies.

Attempts at diplomacy have failed repeatedly, with Pyongyang rejecting working-level talks and experts seeing little realistic chance they can be convinced to give up their nukes.

North Korea conducted its first nuclear test in 2006, and carried out its sixth and most powerful one in September 2017, with growing concerns that it is preparing another test as it attempted to develop tactical nuclear warheads.

A report published this year by the US Congressional Research Service cited external estimates that North Korea already possessed enough material for "20 to 60 warheads".

North Korea is actively pursuing the development of smaller warheads to fit a variety of delivery systems, it said.