A North Korean agency made a threat Thursday that Pyongyang could stage an "unimaginable" strike on the United States at an unexpected time amid tensions over the North's nuclear and missile programs.

The North's intimidation came as the South Korean and US navies are staging massive joint drills in waters off the Korean Peninsula with the US aircraft carrier USS Ronald Reagan being involved.

North Korea's emergency committee for opposing nuclear war drills condemned the Seoul-Washington's move to mobilize nuclear strategic assets near the peninsula.

"The US is running amok by introducing under our nose the targets we have set as primary ones. The US should expect that it would face unimaginable strike at an unimaginable time," the agency said in a statement carried by the Korean Central News Agency in English.

Tensions have heightened amid North Korea's nuclear and missile tests and exchanges of inflammatory rhetoric between US President Donald Trump and the North Korean leadership.

North Korean Kim Jong-un has vowed to take the "highest-level" measures against Washington as Trump threatened to "totally destroy" the North if the US is forced to defend itself and its allies due to Pyongyang's nukes.

North Korea fired two intercontinental ballistic missiles in July and conducted the sixth and most powerful nuclear test last month. (Yonhap)