North Korea said Tuesday that it plans to conduct tests of nuclear warheads and ballistic missiles "in a short time," in an apparent show of defiance in the wake of tougher U.N. sanctions.

The threats add weight to speculation that the North may carry out more nuke and missile provocations following its nuclear test and long-range rocket launch earlier this year.

North Korean leader Kim Jong-un said that "a nuclear warhead explosion test and a test-fire of several kinds of ballistic rockets able to carry nuclear warheads will be conducted in a short time to further enhance the reliance of nuclear attack capability," according to the Korean Central News Agency.

He made such remarks while overseeing an environmental simulation for re-entry technology, which is needed to bring a nuclear-armed ballistic missile back into the atmosphere, according to the report.

"We have proudly acquired the re-entry technology, possessed by a few countries styling themselves as military powers ...,thus making a great progress in the ballistic rocket technology that helps strengthen the independence of the country's defense capability and munitions industry," it said.

Earlier this month, the United Nations Security Council adopted a new sanctions resolution punishing the North for its Jan. 6 nuclear test and Feb. 7 long-range rocket launch.

The North said its long-range rocket launch was to send a satellite into orbit, but Seoul and Washington view it as a cover for a banned test of ballistic missile technology.

Experts say long-range rockets and intercontinental ballistic missiles are basically the same, differing only in payload.

In recent days, the North has ratcheted up its belligerent rhetoric, threatening to make "pre-emptive attacks" against Seoul and Washington at a moment's notice.

Last week, the North's leader ordered his officials to get ready to carry out nuclear attacks and conduct more nuclear tests. He also vowed to "make nuclear strikes at the enemies from anywhere on the ground, in the air, at sea and underwater."

Kim also said his country has made nuclear warheads small enough to fit on ballistic missiles in an apparent threat to Seoul and Washington, which recently began their annual joint military drills. This year's two-month-long exercise mobilizes some 300,000 local troops and 17,000 U.S. forces.

North Korea has made efforts to develop a nuclear-tipped missile capable of reaching as far as the U.S. mainland, and the communist country has insisted that it has entered the technological phase to miniaturize and even diversify its nuclear bombs.

But Seoul has raised doubts about the North's claim, saying there's no evidence that Pyongyang has mastered the required technology to miniaturize nuclear weapons.

Mastering missile re-entry technology and the ability to make smaller nukes would allow North Korea to fire off nuclear-tipped intercontinental ballistic missiles which can target the U.S. mainland.

But South Korea also rejected North Korea's claim that it has acquired missile re-entry technology.

"Seoul believes that the North has yet to secure such technology," said a military official, asking not to be named. "North Korea's claim is seen as aiming to flex its military muscles."

Analysts said that the North is showing its intent not to give up its nuclear weapons program, raising speculation that it may carry out high-profile provocations such as a fifth nuke test or a long-range rocket launch.

"North Korea may push ahead with another nuclear test by detonating a stronger fission bomb before it holds its key party congress in May," said Cheong Seong-chang, a senior researcher at the Sejong Institute.

North Korea is beefing up its efforts to prepare for its ruling party's first congress in more than three decades in May. Pyongyang may be trying to enhance internal solidarity ahead of the party event.

The North conducted nuclear tests in 2006, 2009 and 2013, along with its latest taking place in January. (Yonhap)