The missiles were fired towards the East Sea, also known as the Sea of Japan, just before midnight, South Korea's military said, adding it was still assessing the type of missiles launched by Pyongyang, Yonhap reported early Thursday.

"While strengthening our monitoring and vigilance against additional provocations, our military is maintaining a full readiness posture in close cooperation with the United States," the JCS said in a text message sent to reporters.

The US-South Korea annual Ulchi Freedom Shield exercises always infuriate Pyongyang, which has conducted a record number of weapons tests this year.

The combined air drills involved at least one US B-1B strategic bomber flying above the Korean Peninsula earlier Wednesday, according to Yonhap.

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 Jong Un 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 Tuesday.

"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," the Korean Central News Agency quoted Kim 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."