North Korea has said it fired two short-range ballistic missiles as part of a "tactical nuclear strike drill" prompted by US-South Korean military exercises, state media reported Thursday.

The missile launches -- first reported by the South Korean military -- came amid Washington and Seoul's annual Ulchi Freedom Shield exercises, which always infuriate Pyongyang.

The North's army said in a statement that the missiles were fired late Wednesday in a "tactical nuclear strike drill simulating scorched earth strikes at major command centers and operational airfields" across the border in South Korea.

The "tactical ballistic missiles" were fired towards the East Sea, also known as the Sea of Japan, just before midnight, South Korea's Yonhap news agency quoted Seoul's military as saying.

"The drill is aimed to send a clear message to the enemies," the army said in the statement, carried by the official Korean Central News Agency.

Pyongyang has conducted a record number of weapons tests this year.

The North also staged its own command-level army drills on Tuesday in response to the US-South Korean exercises, during which the country's leader Kim Jong Un visited a training command post, KCNA said.  

"The drill is aimed at letting all the commanding officers and staff sections of the entire army make full preparations for war," KCNA said of the training exercise.

South Korea's Joint Chiefs of Staff meanwhile said the military south of the border was "maintaining a full readiness posture in close cooperation with the United States". 

Their combined air drills involved at least one US B-1B strategic bomber flying above the Korean Peninsula earlier Wednesday, according to Yonhap -- a detail that particularly annoyed Pyongyang.

The North called the overflight "a serious threat" and "pursuant to the scenario for a pre-emptive nuclear strike at the DPRK," the official acronym for North Korea.

- 'Irreversible' nuclear power -

US national security spokesman John Kirby, who was giving a briefing when news of the launch broke, declined to comment to reporters in Washington.

On Tuesday, the United States, South Korea and Japan also held a trilateral naval missile defence exercise that enraged North Korea.

Washington, Seoul and Tokyo have beefed up their defence cooperation in recent months in response to increasing missile provocations by the North.

Last week, Pyongyang carried out its second attempt to put a spy satellite into orbit, although it ended in failure.

Kim has declared North Korea an "irreversible" nuclear power and has called for ramped-up arms production, including tactical nuclear weapons.

He has also called for boosting North Korea's navy, saying the country's waters brimmed with "the danger of a nuclear war", state media reported.

"Owing to the reckless confrontational moves of the US and other hostile forces, the waters off the Korean Peninsula have been reduced into the world's biggest war hardware concentration spot," KCNA quoted Kim as saying.

"To achieve the successes in rapidly developing the naval force has become a very urgent issue in view of the enemies' recent aggressive attempts."

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