Relations between the two Koreas are at one of their worst points in decades, with the nuclear-armed North conducting ever more provocative banned weapons tests as Seoul moved to ramp up security cooperation with Washington in response.

Last year, Kim Jong Un's regime declared North Korea an "irreversible" nuclear power and vowed to exponentially increase weapons production, including tactical nuclear weapons, as the US looks to move more assets to the region to defend ally Seoul.

Seoul's Joint Chiefs of Staff said that it "detected the launch of one short-range ballistic missile from the North's western port city of Nampo at 6:20 pm (0920 GMT)".

It added it was analysing "the possibility that North Korea fired multiple short-range ballistic missiles from the same region simultaneously".

"Our military maintains a full readiness posture while closely cooperating with the US as we have strengthened surveillance and vigilance," it said.

North Korea has long claimed its nuclear weapons and missile programmes are for self-defence, and has bristled over US-South Korea military exercises, describing them as rehearsals for an invasion.

Earlier this week, North Korea accused the US of "intentionally" ramping up tensions, and Kim's powerful sister warned that if the US were to intercept one of Pyongyang's missile tests, it would be seen as a "clear declaration of war".

- 'Freedom Shield' -

After talks between Kim and then-US president Donald Trump collapsed in 2019, diplomacy has stalled and the North has doubled down on military development.

South Korea's hawkish President Yoon Suk Yeol has moved to boost diplomatic ties and security cooperation with Tokyo and Washington in response to growing threats from Pyongyang.

US President Joe Biden will host Yoon for a state visit on April 26, and the South Korean leader will also visit Tokyo next week, his office said.

This month, the US and South Korean militaries will hold their largest joint drills in five years.

Ahead of those exercises, named "Freedom Shield" and scheduled for at least 10 days starting March 13, the allies held air drills this week featuring a nuclear-capable US B-52 heavy bomber.

"This is likely only the beginning of a series of provocative tests by North Korea," said Leif-Eric Easley, a professor at Ewha University in Seoul.

"Pyongyang is poised to respond aggressively to major US-South Korea defence exercises, as well as to President Yoon's upcoming summits with (Japan's Prime Minister Fumio Kishida and US President Joe Biden)," he said.

"The Kim regime may order missile firings of longer ranges, attempt a spy satellite launch, demonstrate a solid-fuel engine, and perhaps even conduct a nuclear test."

North Korea has framed its missile tests and military drills as justified countermeasures following US-South Korea drills.

Last week, it called on the United Nations to urge a halt to these exercises, and reiterated that its nuclear weapons ensured the balance of power in the region.