North Korea said Monday it is not frightened by US threats of possible pre-emptive military action to halt its nuclear and missile buildup.

A spokesman for North Korea’s Foreign Ministry slammed US Secretary of State Rex Tillerson’s recent talk of tougher sanctions, more pressure, and possible military action, and said the North would not be deterred in its nuclear programme.

“The nuclear force of [North Korea] is the treasured sword of justice and the most reliable war deterrence to defend the socialist motherland and the life of its people,” the official Korean Central News Agency quoted the spokesman as saying.

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Tillerson recently visited Japan, South Korea and China on trip that focused on North Korea’s nuclear programme. On Friday, he signaled a tougher strategy that left open the possibility of pre-emptive military action.

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“Let me be very clear: The policy of strategic patience has ended,” he said after visiting the heavily militarised border between the rival Koreas.