Pyongyang has doubled down on upgrading its weapons programme this year despite facing crippling economic sanctions.

"North Korea fired an unidentified ballistic missile(s) into the East Sea," Seoul's Joint Chiefs of Staff said Sunday morning, referring to the Sea of Japan.

The Japanese Coastguard said "possible ballistic missile(s)" had been launched from North Korea, citing the defence ministry.

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

It was the allies' first joint military exercise since South Korea's President Yoon Suk-yeol took office last month, and their first joint drills involving an aircraft carrier since November 2017.

Pyongyang has long protested against Seoul and Washington's joint military exercises, calling the drills as 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," the JCS said in a statement.

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

- Nuclear test -

Last month, Pyongyang test-fired three missiles, including possibly its largest intercontinental ballistic missile, the Hwasong-17.

That launch came 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.

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 diplomacy, which collapsed in 2019.

But Pyongyang abandoned this self-imposed moratorium on long range and nuclear tests, and has carried 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.