North Korean dictator Kim Jong Un threatened Friday to expand his nuclear arsenal with more sophisticated atomic weapons after indicating earlier in the week that he desired improved relations with other nations, according to reports.

During a key ruling party meeting Friday, Kim said the fate of diplomacy with the United States would depend on whether America abandons hostile policies toward the communist nation.

The announcement was seen as an effort to pressure the incoming administration of President-elect Joe Biden, whose inauguration is planned for later this month.

During the last four years, Kim gained some unprecedented inroads with President Donald Trump, who leaves office Jan. 20, but both leaders failed to reach any meaningful agreement on controlling nuclear weapons in the country.

Kim said Friday he won’t use his nuclear weapons unless “hostile forces” intend to use their nuclear weapons against North Korea first.

But he added that North Korea must further strengthen its military and nuclear capability as the danger of a U.S. invasion on North Korea increases.

Kim ordered officials to develop missiles with multiple warheads, underwater-launched nuclear missiles, spy satellites and nuclear-powered submarines, The Associated Press reported.

He also authorized the production of a solid-fuel intercontinental ballistic missile that can be launched from sea and land, reports said.

Kim’s order was made during a fourth-day meeting Friday of his country’s first ruling party congress in five years, the AP reported.

Sung-Yoon Lee, an associate professor at Tufts University and expert on the region, also wrote on Twitter, that Kim has called for the production of “smaller, lighter nuclear bombs ... more ultra-big nuclear warheads” and “advancing capabilities in accurate preemptive nuclear strike & second-strike capabilities on targets 15,000km away.”

Kim, who celebrated his 37th birthday Friday, has not yet publicly acknowledged Biden’s victory. Friday was the leader’s first substantive announcement about his country’s weapons program since his government introduced a new intercontinental ballistic missile during a military parade in October.

Kim’s development plan for the next five years also includes “comprehensively expanding and developing the external relations” with the U.S., South Korea, as well as other foreign powers such China and Russia, the leader said during meetings of the ruling Workers’ Party of Korea in Pyongyang, according to Bloomberg, which cited the official Korean Central News Agency.