Military tensions on the Korean peninsula have risen sharply this year as the North has carried out an unprecedented blitz of weapons tests, including the launch of its most advanced intercontinental ballistic missile ever last month.

"North Korea fires an unidentified ballistic missile into (the) East Sea," South Korea's Joint Chiefs of Staff said Saturday, referring to the body of water also known as the Sea of Japan.

The Monday intrusion by the drones -- one of which flew close to the capital Seoul -- prompted the South's military to deploy fighter jets and attack helicopters.

But despite a five-hour operation, the military failed to shoot down a single drone, prompting widespread criticism over the response and an apology from the country's defence minister.

South Korean president Yoon Suk-yeol said the incident was "intolerable" and added that the South should ensure that Pyongyang realised that "provocations are always met with harsh consequences".

Seoul's military subsequently staged drills Thursday that the country's top brass said would improve its defences against any future drone provocations.

Monday marked the first time in five years that North Korean drones had flown into the South's airspace.

- Party meeting -

Earlier this year, North Korean leader Kim Jong Un said he wanted his country to have the world's most powerful nuclear force and declared the North an "irreversible" nuclear state.

North Korea is currently holding a major party meeting in Pyongyang at which Kim and other senior party officials are outlining their policy goals for 2023 in key areas including diplomacy, security and the economy.

On Wednesday, Kim set out "new key goals" for the country's military, state media reported at the time, without giving any specifics.

North Korea's end-of-year plenary meetings are typically used by the regime to unveil the country's domestic and foreign policy priorities for the year ahead.

Full details of the current plenary are expected to be announced after it concludes.

While Kim focused on the economy at the 2021 plenary, analysts are widely expecting a shift in tone to highlight the military front this year in light of the recent blitz of missile launches.

In past years, Kim had delivered a speech every January 1, but he recently dropped the tradition in favour of making announcements at the year-end plenary meeting.

The United States and South Korea have warned for months that Pyongyang is preparing to conduct its seventh nuclear test.

North Korea is under multiple UN Security Council sanctions over its nuclear and missile activity since 2006.