North Korea has said the salvo of four missiles it fired on Monday was a rehearsal for a strike on US military bases in Japan.

In a brazen declaration of its targets, the state-run Korean Central News Agency said the drill involved artillery units "tasked to strike the bases of the US imperialist aggressor forces in Japan in a contingency".

About 54,000 US military personnel are stationed across 85 facilities in Japan, according to US Department of Defence data.

Japanese Prime Minister Shinzo Abe told US President Donald Trump yesterday that, with North Korea's threat in "a new phase", Japan will move quickly to assume a "larger role and responsibility to enhance the deterrence under the US-Japan alliance".

Mr Trump, in turn, said the US stands by its ally "100 per cent".

Admiral Yutaka Murakawa, chief of staff of Japan's Maritime Self-Defence Force, said North Korea's capability "is believed to be improving with every new launch".

Experts told The Straits Times that the launch showcased Pyongyang's precision, as well as capability to overwhelm regional missile defence systems.

"It is the largest number of missiles launched simultaneously, and is a simulation of a saturation attack," said Dr Narushige Michishita of the National Graduate Institute for Policy Studies.

Research fellow Ippeita Nishida of Tokyo think-tank Sasakawa Peace Foundation noted the North has demonstrated a theoretical capability to "deliver a sudden strike".

"This is a concern," said Dr Tosh Minohara of Kobe University. "They can launch rockets and we won't know about it until after the launch."

Japan's Defence Minister Tomomi Inada said yesterday more launches are possible during the ongoing annual US-South Korea military drills. Pyongyang last year fired seven missiles over the two-month exercise.

The United Nations Security Council will convene an emergency meeting in New York at 10am today (11pm in Singapore).