International Atomic Energy Agency (IAEA) head Rafael Grossi warned that North Korea's nuclear program is going "full steam ahead" in violation of United Nations resolutions.
The program includes the enrichment of uranium and plutonium separation, Grossi said in Vienna on Monday.
"In the Democratic People’s Republic of Korea, the nuclear program goes full steam ahead," Grossi said at the beginning of the annual meeting of the IAEA member states.
In doing so, Pyongyang is violating several U.N. resolutions.
IAEA inspectors have no access to the isolated country, which has already carried out several tests for the development of nuclear weapons.
The Vienna-based atomic authority uses satellites to observe the nuclear facilities.
In the run-up to the conference, the IAEA reported signs that North Korea had restarted the nuclear reactor in its controversial Yongbyon nuclear center.
The small reactor can supply plutonium for making atomic bombs. Nuclear warheads can be built with enriched uranium.
A week ago, North Korea reported the successful test of strategically important missiles and indicated that the guided missiles were also being developed for nuclear warheads.
For years, Pyongyang has been developing missiles that are capable of hitting not only South Korea and Japan but that can carry warheads as far as the United States.
North Korea has declared itself a nuclear power. According to estimates by the U.S.-based Arms Control Association from August 2020, there are 30 to 40 nuclear warheads in North Korea.
Also on Monday, the reclusive state sharply criticized Washington’s new security alliance with the U.K. and Australia in the Indo-Pacific region.
The pact and the U.S. decision to support Australia in acquiring nuclear-powered submarines are "extremely undesirable and dangerous actions," state media cited an unnamed official as saying.
The behavior of the U.S. would "overturn the strategic balance in the Asia-Pacific region and set in motion a race for nuclear weapons," the official added.
He warned that Pyongyang would take countermeasures if the move was found to have an impact on North Korea’s security.
The AUKUS alliance between Australia, the U.K. and the U.S. will see Canberra acquire nuclear-powered submarines from the U.S. in the next 18 months.