The Democratic People's Republic of Korea has adopted a constitutional amendment to enshrine its policy on nuclear force, state media reported on Thursday, as the country's leader pledged to accelerate production of nuclear weapons to deter what he called U.S. "provocations."

DPRK's parliament, the Supreme People's Assembly, unanimously adopted "the crucial agenda item for formulating the DPRK's policy on the nuclear force as the basic law of the state," KCNA said.

Addressing the parliament, the country's leader, Kim Jong Un, said it was important to "accelerate the modernization of nuclear weapons in order to hold the definite edge of strategic deterrence," KCNA reported. 

The amendment comes a year after Pyongyang officially enshrined in law the right to use preemptive nuclear strikes to protect itself. 

Kim urged officials to "further promote solidarity with the nations standing against the U.S. and the West's strategy for hegemony," denouncing trilateral cooperation between the U.S., South Korea and Japan as the "Asian-version NATO."

"This is just the worst actual threat, not threatening rhetoric or an imaginary entity," he said. 

Kim returned home last week from a rare trip to Russia, during which he and Russian President Vladimir Putin agreed to boost military cooperation. U.S. and South Korean officials have expressed concern.

On Tuesday, South Korean President Yoon Suk Yeol warned Pyongyang against using nuclear weapons, as Seoul put on the first large-scale military parade in a decade, with weapons ranging from ballistic missiles to tanks rolling through Seoul in a show of force. 

The announcement by parliament also comes after DPRK state media said on Wednesday that Pyongyang had decided to expel Private Travis King, the U.S. soldier who ran into the country in July. The U.S. said he is now in American custody and heading home after being expelled into China.

Also on Tuesday, United Nations Secretary-General Antonio Guterres warned at the general assembly against a new atomic arms race bringing the threat of "annihilation" to the world. The UN chief called for a revitalized push to reduce and eventually eliminate those weapons.

(With input from Reuters)

(Cover: The national flag of the DPRK is seen in Geneva, Switzerland, June 20, 2017. /Reuters)