North Korea is pressing on with its nuclear weapons program and several countries believe it has "probably developed miniaturized nuclear devices to fit into the warheads of its ballistic missiles," according to a confidential U.N. report.

The report by an independent panel of experts monitoring U.N. sanctions said the countries, which it did not identify, believed North Korea's past six nuclear tests had likely helped it develop miniaturized nuclear devices. Pyongyang has not conducted a nuclear test since September 2017.

The interim report, seen by Reuters, was submitted to the 15-member U.N. Security Council North Korea sanctions committee on Monday.