North Korea takes action on their threatening

North Korea conducted a big live-fire exercise on Tuesday to mark the foundation of its military as a U.S. submarine docked in South Korea in a show of force amid growing concern over the North's nuclear and missile programs.

The port call by the USS Michigan came as a U.S. aircraft carrier strike group steamed toward Korean waters and as top envoys for North Korea policy from South Korea, Japan and the United States met in Tokyo.

Fears have risen in recent weeks that North Korea would conduct another nuclear test or long-range missile launch in defiance of U.N. sanctions, perhaps on the Tuesday anniversary of the founding of its military.

But instead of a nuclear test or big missile launch, North Korea deployed a large number of long-range artillery units in the region of Wonsan on its east coast for a live-fire drill, South Korea's military said. North Korea has an air base in Wonsan and missiles have also been tested there.

"North Korea is conducting a large-scale firing drill in Wonsan areas this afternoon," the South's Office of Joint Chiefs of Staff said in a statement.

The South Korean military was monitoring the situation and "firmly maintaining readiness", it said. North Korea's state media was defiant in a commentary marking the 85th anniversary of the foundation of the Korean People's Army, saying its military was prepared "to bring to closure the history of U.S. scheming and nuclear blackmail".

North Korea's growing nuclear and missile threat is perhaps the most serious security challenge confronting U.S. President Donald Trump. He has vowed to prevent North Korea from being able to hit the United States with a nuclear missile and has said all options are on the table, including a military strike.