Pyongyang has doubled down on upgrading its weapons programme this year, despite facing crippling economic sanctions, with officials and analysts warning that the regime is preparing to carry out a fresh nuclear test.

"Our military detected eight short-range ballistic missiles fired by North Korea," Seoul's Joint Chiefs of Staff said.

The missiles were launched from multiple locations over a 30-minute period, including Sunan in capital Pyongyang, Tongchang-ri in North Pyongan province, and Hamhung in South Hamgyong province, they said.

They travelled different distances -- from 110 kilometres (68 miles) to 670 kilometres -- and flew at different altitudes of up to 90 kilometres, it added.

Japanese Defence Minister Nobuo Kishi said the simultaneous test-firing from multiple locations was "unusual".

"This is absolutely unacceptable," he added.

According to local reports, two missiles were shot from each site, likely from transporter erector launchers (TELs) -- the largest number of ballistic missiles North Korea has recently launched on a single day and occasion.

Analysts say the volley of missile launches Sunday -- one of nearly 20 weapons tests by Pyongyang so far this year -- is a pointed message for Seoul and Washington.

"It shows North Korea's intention to neutralise the missile defence system of South Korea and the United States with multiple simultaneous attacks during emergency," said Cheong Seong-jang, a researcher at the Sejong Institute.

The move comes barely a day after South Korea and the United States wrapped up large-scale, three-day exercises involving the USS Ronald Reagan, a 100,000-tonne nuclear-powered aircraft carrier.

The exercises were the allies' first joint military drills since South Korea's hawkish new President Yoon Suk-yeol took office last month, and the first involving an aircraft carrier since November 2017.

Pyongyang has long protested against the joint exercises, calling them rehearsals for invasion.

"The exercise consolidated the two countries' determination to sternly respond to any North Korean provocations while demonstrating the US commitment to provide extended deterrence," Seoul's Joint Chiefs of Staff said in a statement.

Go Myong-hyun, a researcher at the Asan Institute for Policy Studies, said Sunday's launch was likely a response to the US-South Korea manoeuvres.

"It seems that they fired eight missiles because the scale of the joint drills has expanded in their view," he told AFP.

- Nuclear test -

Last month, during a summit with Yoon, US President Joe Biden said Washington would deploy "strategic assets" to the South if necessary as part of efforts to bolster deterrence.

Pyongyang test-fired three missiles, including possibly its largest intercontinental ballistic missile, the Hwasong-17, just days after Biden left South Korea following his summit with Yoon.

US and South Korean officials have warned for weeks that Pyongyang may conduct a seventh nuclear test.

Last month, a US bid to impose fresh UN sanctions on Pyongyang over its missile launches was vetoed by Russia and China.

Despite struggling with a recent Covid-19 outbreak, North Korea has resumed construction on a long-dormant nuclear reactor, new satellite imagery has indicated.

South Korea's presidential office said last month that Pyongyang had carried out tests of a nuclear detonation device in preparation for its first nuclear test since 2017.

Long-range and nuclear tests have been paused since North Korean leader Kim Jong Un met then-US president Donald Trump for a bout of high-profile negotiations that collapsed in 2019.

But Pyongyang has since abandoned this self-imposed moratorium, carrying out a blitz of sanctions-busting weapons tests this year, including firing an intercontinental ballistic missile (ICBM) at full range.

Analysts have warned Kim could speed up nuclear testing plans to distract North Korea's population from the disastrous coronavirus outbreak.