The United States said it would soon send a missile defense system to Guam to defend it from North Korea, as the U.S. military adjusts to what Defense Secretary Chuck Hagel called a "real and clear danger" from Pyongyang, according to Reuters.

Hours later, South Korea's Yonhap news agency said North Korea had moved what appeared to be a mid-range Musudan missile to its east coast. It was not clear if the North planned to fire the rocket or was just putting it on display as a show of force, one South Korean government source was quoted as saying.

North Korea also barred entry to a joint industrial complex it shares with the South for a second day on Thursday, April 4 and said it would shut the zone if Seoul continued to insult it.

"The assumption remains that this is more bluster ...," said Rob Ryan, a strategist with RBS in Singapore. "But from here, we've reached a level of tensions that say things can't get too much worse without an actual exchange of fire."

North Korea also repeated its threat to launch a nuclear attack on the United States. Pyongyang said it had ratified a potential strike because of U.S. military deployments around the Korean peninsula that it claimed were a prelude to a possible nuclear attack on the North.

Washington had been informed of the potential attack by North Korea, a spokesman for its army said in a statement carried by the English-language service of state news agency KCNA. It was unclear how such a warning was given since North Korea does not have diplomatic ties with Washington.

The report from KCNA appeared to re-state many of the month-long fusillade of threats emanating from Pyongyang.

Experts say North Korea is years away from being able to hit the continental United States with a nuclear weapon, despite having worked for decades to achieve nuclear-arms capability.