North Korea's nuclear arsenal must be readied for deployment at any time, leader Kim Jong-un ordered Friday, days after the U.N. passed the harshest sanctions on his country in over two decades.

North Korean state TV quoted Kim as saying his country needed to shift its strategy to one of "preemptive attack" and that nuclear warheads needed to be "on standby so as to be fired at any moment." Such aggressive rhetoric from the regime is common following international action to punish Pyongyang's violations of international law.

The full extent of the isolated country's nuclear capabilities is unknown, but some experts doubt whether North Korea has the power to mount its nuclear warheads on intercontinental ballistic missiles for action as Kim described. A U.S. defense official told AFP "we have not seen North Korea test or demonstrate the ability to miniaturise a nuclear weapon" so it could be used on an ICBM.

The U.N. Security Council on Wednesday unanimously passed a resolution further punishing North Korea for its continued nuclear activity. The body faced international pressure to act in condemnation of Pyongyang's two tests already this year, one in January it claims was a hydrogen bomb and another launch in February of a satellite using banned missile technology.

The North also launched several short-range missiles into the ocean off its coast Thursday following the passage of the sanctions. The new measures require all cargo going to and leaving from North Korea to be inspected, and places additional economic sanctions on banks, natural resource exports and the import of luxury goods including watches and recreational equipment.

Kim called the sanctions "gangster-like" and the North Korean foreign ministry defended its launches in recent days as "a legitimate exercise of our right to self-defense, which will continue as long as the hostile U.S. policy is in place."