(CNN) North Korea fired four ballistic missiles early Monday morning in what Japan's leader described as "an extremely dangerous action."

A fifth missile failed to launch, a US official told CNN.

Military authorities in South Korea, Japan and the United States all confirmed the launch of four projectiles, which traveled almost 1,000 kilometers (620 miles) towards the Sea of Japan, also known as the East Sea. One US official said they were intermediate-range missiles.

Three landed inside Japan's exclusive economic zone, Prime Minister Shinzo Abe said, which extends 200 nautical miles from its coastline, according to international maritime law.

The launch underscored the rapid evolution of North Korea's weapons program, which experts say has begun moving at a faster rate to develop and deploy missiles.

"They did a launch a month ago, they're now launching (more) in 30 days. That's a third of the time they used to need," said Carl Schuster, a professor at Hawaii Pacific University.

Jeffrey Lewis, director of the US-based East Asia Nonproliferation Program, told CNN the North Koreans' recent behavior was the sort "you see from a state that is planning to deploy nuclear weapons to its military units."

Washington watching

Later on Monday, two US officials told CNN that US intelligence is closely monitoring the underground nuclear test site via satellite amid signs the regime could be preparing for another underground test.

While an underground test is impossible to predict, officials say some signs of activity at the site are similar to those that occurred just before the last test.

A US official also said that Washington is continuing to see activity including missile engine tests at another North Korean launch site, that signal potential additional launches -- which the US widely expects.

The official said the launch did not involve one of North Korea's more advanced missiles in development such as the Musudan missile.

Escalation

Speaking to the Japanese Parliament, Abe said Monday the launch was a clear violation of UN Security Council resolutions.

Experts said the move was almost certainly in reaction to joint military exercises between South Korea and the US, which Pyongyang views as preparations for an invasion.

British Foreign Secretary Boris Johnson also condemned the missile launches, calling them a threat to international peace and security.

The launch took place in Tongchang-ri, in North Korea's North Pyongan province. A spokesperson for South Korea's Joint Chiefs of Staff said the missiles flew as high as 260 kilometers (162 miles).

Speaking to CNN, a US official said a preliminary assessment found that North Korea launched a total of five extended-range SCUD missiles, one of which failed.

The other four landed in the sea, the official said. The mobile missile launches did not involve any of North Korea's more advanced missiles currently in development, such as the Musudan.

'Grave violation'

Acting President Hwang Kyo-ahn told the South Korean National Security Council Monday he strongly condemned North Korea's actions.

"This is a direct challenge to the international community and a grave violation," he said.

"Having seen the brutality of North Korea from Kim Jong Nam, I'd say the consequences of the Kim Jong Un regime having nuclear weapons will be horrible," he said, referring to the killing of the North Korean leader's estranged half brother at Kuala Lumpur airport last month. North Korea has denied any involvement in his death.

The US State Department said it remained "prepared to use the full range of capabilities at our disposal against this growing threat."

"The DPRK's provocations only serve to increase the international community's resolve to counter the DPRK's prohibited weapons of mass destruction programs," acting State Department spokesman Mark Toner said in a statement.

Speaking at his regular daily press briefing, Geng Shuang, China's Ministry of Foreign Affairs spokesman, said they opposed the North Korean launch, adding it was in violation of UN resolutions.

"China has also noted that the US and South Korea are holding large-scale military exercises targeting North Korea," he said. "Under current circumstances, all relevant sides should exercise restraint, and avoid actions that would provoke each other and escalate tensions."

Foal Eagle drills

South Korea and United States' annual military exercises, known as Foal Eagle and which both countries say are defensive in nature, began March 1 and are expected to last until April 30.

North Korea has already denounced this year's exercises through state media service KCNA.

"Now that the US imperialists and the South Korean puppet forces again kicked off the dangerous nuclear war drills against the DPRK at its doorstep, our army will counter them with the toughest counteractions," it said.

The exercises usually draw condemnation and retribution from Pyongyang.

"They (are) making it clear that as long as the US continues these exercises, North Korea will keep taking steps to bolster their deterrent," Jeffrey Lewis, Adjunct Professor at the Middlebury Institute of International Studies at Monterey, told CNN.

During last year's drills, North Korea fired multiple short- to medium-range missiles and announced it could place nuclear warheads on its weapons.

North Korean missile tests

This isn't the first time North Korea has launched multiple missiles over this distance.

In September 2016, North Korea launched three ballistic missiles about 1,000 kilometers to land in Japan's Air Defense Identification Zone , provoking a strong response from the country.

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That missile was also fired from North Pyongan province and traveled 500 kilometers (310 miles) before landing in the Sea of Japan.

The launch happened while Abe was in the United States visiting President Donald Trump.

North Korean state media reported that Kim Jong Un personally oversaw the operation.