Analysts have said Seoul's plan to construct one of the atomic-driven vessels would likely draw an aggressive response from Pyongyang.

South Korea's Joint Chiefs of Staff said North Korea fired an unidentified ballistic missile towards the East Sea, referring to the body of water also known as the Sea of Japan.

The missile landed in the sea outside Japan's economic waters and no damage or injuries had been reported, said Japanese Prime Minister Sanae Takaichi.

But the Kremlin defended North Korea's latest launch, saying Pyongyang -- a key ally for Russia during its Ukraine campaign -- had the "legitimate right" to do so.

"We are respectful of the legitimate right of our friends in the DPRK (North Korea) to ensure their security and take measures for it," Kremlin spokesman Dmitry Peskov told reporters.

Washington's security ally Tokyo, meanwhile, said North Korea's ballistic missile launches have been "absolutely unforgivable".

As "evidenced" by North Korea's provocations, "it's never too early to accelerate efforts to revamp our defence capabilities," Defence Minister Shinjiro Koizumi said.

"We will consider what steps are needed to protect our nation's... peace and the lives of our people, without ruling out any options."

The missile launched at 12:35 pm (0335 GMT) from an area north of Pyongyang and flew around 700 kilometres (435 miles), South Korea's military said.

North Korea has significantly increased missile testing in recent years, which analysts say is aimed at improving precision strike capabilities, challenging the United States as well as South Korea, and testing weapons before potentially exporting them to Russia.

"From North Korea's perspective, the possibility of sudden attacks from the East Sea will be a source of anxiety," Ahn Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP.

"If South Korea acquires a nuclear-powered submarine, they would be able to enter North Korean waters and preemptively monitor or intercept weapons such as submarine-launched ballistic missiles."

Trump had announced that South Korea would build the submarine in the United States, but Seoul says it is considering making it at home.

Unlike diesel-powered submarines, which must regularly surface to recharge their batteries, nuclear-powered ones can remain submerged for far longer.

- 'Irreversible' nuclear state -

Analysts say developing a nuclear-powered submarine would be a significant leap for South Korea.

Only the United States, Australia, China, Russia, India, France and Britain have moved toward nuclear-powered submarines, according to media and analysis reports.

Since North Korean leader Kim Jong Un's 2019 summit with Trump collapsed over the scope of denuclearisation and sanctions relief, Pyongyang has repeatedly declared itself an "irreversible" nuclear state.

Kim has since been emboldened by the war in Ukraine, securing critical support from Moscow after sending thousands of troops to fight alongside Russian forces.

Pyongyang did not respond to Trump's offer to meet with Kim last week, and instead its Foreign Minister Choe Son Hui headed to Moscow, where she and Russian President Vladimir Putin agreed to strengthen bilateral ties.

In September, Kim appeared alongside Chinese President Xi Jinping and Putin at an elaborate military parade in Beijing -- a striking display of his new, elevated status in global politics.

Trump met Kim three times during his first term and once famously said the pair had fallen "in love", but the US leader ultimately failed to secure a lasting agreement on North Korea's nuclear programme.

South Korean lawmaker Lee Seong-kweun said this week that Seoul's spy agency believes Kim was still open to talks with Washington, "and will seek contact when the conditions are in place".

Although the proposed meeting with Trump did not materialise, "multiple signs suggest" that Pyongyang "had been preparing behind the scenes for possible talks with the US", said the lawmaker.