The chairman of the Supreme People's Assembly of North Korea, Park In-chol, accused the United States and South Korea of preparing to "implement" nuclear war, Yonhap reported.
He accused Western countries of causing "instability and chaos" to maintain their "hegemonic position".
He insisted that the country's nuclear development was for self-defense purposes, vital to ensuring its security against such threats.
Park In-cheol made the statement at the 6th World Conference of Speakers of Parliaments, led by the United Nations, in Switzerland.
"The advanced preparations of the United States and the Republic of Korea for carrying out a nuclear war have entered a serious stage, Park said. This forces the DPRK to take all necessary measures in response to such a geopolitical crisis. "Having the ability to proactively deal with various security threats is a matter of vital importance for our country to protect its sovereignty, security threats and territorial integrity," he said.
The North will never tolerate the "freedom and arbitrariness of the US and its forces" and will "fully fulfill its responsibility" to uphold international justice, he vowed.
Pak also said his country was carrying out various activities under its development roadmap for "prosperity and happiness" to his people, including long-term construction plans and other measures to improve education, healthcare and science.
"Our people are achieving miraculous successes in their struggle for all-round development," Park said.
South Korean National Assembly Speaker Woo Won-shik attended the meeting and delivered a speech calling for maintaining open channels of communication between the two Koreas even as inter-Korean relations remain strained.
No meeting has taken place between Woo and Park.
A potential meeting between the leaders of North Korea and the United States will remain a "hope" for Washington, if the White House does not change its attitude towards Pyongyang and does not recognize the DPRK as a nuclear-weapon state.
This was stated a day earlier by the sister of the leader of the DPRK Kim Jong-un - Kim Yo-jong.
She recalled that it is now 2025, not 2018 or 2019.
"Recognition of the irreversible status of our state - a nuclear weapons holder, its capabilities and the so clear fact that the geopolitical situation has changed radically, should be a prerequisite for all further assumptions and reasoning," the sister of the North Korean leader noted.
According to her, any attempts to deny the nuclear status of the DPRK will be rejected, and the establishment of ties between the heads of state with the aim of denuclearizing Pyongyang will be considered as "mockery".
Earlier, a White House official told the "Yonhap" agency that US President Donald Trump remains open to engaging with North Korean leader Kim Jong Un to achieve the denuclearization of the DPRK.