SEOUL, South Korea — The North Korean leader, Kim Jong-un, has urged his military to have its nuclear warheads deployed and ready to be fired at any moment, the country’s state-run news agency reported Friday.

Mr. Kim’s comments were reported a day after the United Nations Security Council approved tougher sanctions aimed at curtailing his country’s ability to secure funds and technology for its nuclear weapons and ballistic-missile programs.

The North’s official Korean Central News Agency called the resolution unanimously adopted by the Council “unprecedented and gangster-like,” and it quoted Mr. Kim as repeating his exhortation to his military to further advance its nuclear and missile capabilities.

“The only way for defending the sovereignty of our nation and its right to existence under the present extreme situation is to bolster up nuclear force, both in quality and quantity, and keep balance of forces,” Mr. Kim was quoted as saying.