Pyongyang has in recent weeks dramatically ramped up missile launches and military exercises, which it now describes as "tactical nuke" drills, as Seoul and Washington say leader Kim Jong Un is close to conducting what would be his country's seventh nuclear test.

North Korea's military said in a rare statement that its latest actions came in response to a "provocative" South Korean artillery exercise near the border.

The Korean People's Army "took strong military countermeasures", according to a statement carried by the official Korean Central News Agency early Friday.

Pyongyang issued "a stern warning to the South Korean military inciting military tension in the frontline area with reckless action", said the statement.

South Korea's military said it had detected the ballistic missile launch from the Sunan area in Pyongyang early Friday, just hours after Pyongyang flew 10 fighter jets close to the border.

The North Korean jets crossed a Seoul-set "reconnaissance line" which triggers an automatic operational response. Seoul then scrambled military aircraft, including F-35A fighter jets, the Joint Chiefs of Staff said.

North Korea also fired some 170 artillery shots into waters off its east and west coasts, violating a maritime "buffer zone" agreed in a 2018 deal, the JCS said.

Later Friday, Seoul said it had detected about 80 additional artillery shells fired by North Korea into the buffer zone.

"Firing artillery shots in the East and West Sea buffer zones is a clear violation of the September 19 Military Agreement," the JCS said in a statement, referring to the 2018 deal.

"We strongly warn them and urge them to stop immediately."

There have been growing calls from ruling-party parliamentarians for South Korea's hawkish new administration to scrap the 2018 agreement.

Pyongyang firing into the buffer zone could be in protest at this, Hong Min of the Korea Institute for National Unification told AFP.

"The fact that the shells fell in the buffer zone, rather than across the border, indicate the North isn't willing to go all the way to drop the agreement but rather an attempt to test how Seoul will respond to it."

- New warning? -

South Korea on Friday imposed its first unilateral sanctions in five years, targeting North Korean individuals and institutions.

The United States also condemned the Friday ballistic missile launch, saying that it -- like the many other recent launches -- was in violation of multiple UN sanctions.

"We continue to seek serious and sustained dialogue with the DPRK, but the DPRK refuses to engage," a State Department spokesperson said, referring to the North by its official name.

Earlier this week, Kim Jong Un dismissed the idea of restarting talks over its banned weapons programmes, saying North Korea "felt no necessity to do so".

The country revised its nuclear laws last month to allow preemptive strikes, with Kim declaring North Korea an "irreversible" nuclear power -- effectively ending the possibility of negotiations over its arsenal.

Since then, Seoul, Tokyo and Washington have ramped up combined military exercises, including deploying a nuclear-powered US aircraft carrier to the area twice, infuriating Pyongyang, which sees such drills as rehearsals for invasion.

- Fears of nuclear test -

The overnight barrage follows the test-firing of two long-range strategic cruise missiles on Wednesday, which itself came two days after the North said it had wrapped up two weeks of "tactical nuclear" drills.

Kim expressed "great satisfaction" with the recent tests, which he said showed the country's nuclear combat forces were at "full preparedness for actual war", state media reported.

With talks long stalled -- and Ukraine-linked gridlock at the United Nations stymying fresh sanctions -- Kim has doubled down on developing and testing his banned nuclear arsenal.

Officials in Seoul and Washington have been warning for months that Pyongyang is ready to conduct another nuclear test -- which would be the country's seventh.

Kim has said North Korea will "focus all efforts on the endless and accelerating development of the national nuclear combat armed forces".

He made acquiring tactical nukes -- smaller, lighter weapons designed for battlefield use -- a top priority at a key party congress in January 2021.