South Korea scrambled about 80 military aircraft, including advanced F-35 fighter jets, on Friday after tracking about 180 flights by North Korean warplanes inside North Korean territory in what appeared to be a defiant show of strength.

North Korea’s mobilisation of warplanes came after it test-fired around 30 ballistic missiles over the previous two days, including an intercontinental ballistic missile on Thursday that triggered evacuation warnings in Japan, in an angry response to ongoing joint exercises by hundreds of US and South Korea military planes.

Later on Friday, North Korea’s Foreign Ministry called its military actions this week an appropriate response to the joint exercises, which it described as a display of US “military confrontation hysteria”. It said North Korea will respond with the “toughest counteraction” to any attempts by “hostile forces” to infringe on its sovereignty or security interests.

South Korea’s Joint Chiefs of Staff said the North Korean warplanes were detected in various areas inland and along the country’s eastern and western coasts but did not come particularly close to the countries’ shared border. The South Korean military spotted about 180 flight trails from 1pm to 5pm, but it wasn’t immediately clear how many North Korean planes were involved and whether some may have flown more than once.

None of the planes breached the South Korean military’s virtual “tactical action” line, which is 20km to 50km north of the two countries’ land and sea boundaries and is used for monitoring purposes to give South Korea enough time to respond to provocations or attacks.

South Korea still scrambled about 80 of its own warplanes, including an unspecified number of F-35 fighters. There weren’t any immediate reports of clashes.

US and South Korean forces have been conducting joint “Vigilant Storm” aerial exercises involving about 240 warplanes, including F-35s. The exercises had been scheduled to end Friday, but the allies extended them to Saturday in response to North Korea’s intensified testing activity this week.

The extension of the drills was announced on Thursday after North Korea test-fired an ICBM, which triggered evacuation alerts and temporarily halted trains in northern Japan. It followed the launch by firing two short-range ballistic missiles into the sea.

A South Korean Air Force F15K fighter jet takes off in an undisclosed location in South Korea. Photo / AP

Pak Jong Chon, a senior North Korean military official, then issued a statement threatening retaliation over the drills’ extension. North Korea subsequently fired three additional missiles into the sea and shot around 80 artillery rounds into the eastern parts of maritime buffer zones created by the rivals off their eastern and western coasts in 2018 as part of agreements to reduce tensions.

On Wednesday, North Korea fired more than 20 missiles, the most it has launched in a single day. Those launches came after Pak issued a veiled threat of a nuclear conflict with the United States and South Korea over their joint drills, which the North says are rehearsals for a potential invasion.

After already setting an annual record with dozens of ballistic missile launches in 2022, North Korea has further dialled-up its testing activity since late September, including what it described as simulated nuclear attacks on South Korean and US targets. It has said its tests are meant as a warning against the United States military drills with allies South Korea and Japan.

In response to North Korea’s increased weapons tests and growing nuclear threat, the US and South Korea this year have resumed their large-scale military exercises that were downsized or suspended in previous years to support the Trump administration’s diplomatic efforts with the North and because of the pandemic.

North Korea particularly despises Vigilant Storm, which includes US F-35B stealth fighter jets that can be heavily armed for land strike missions, said Hong Min, a senior analyst at Seoul’s Korea Institute for National Unification. He said the barrage of North Korean missile and artillery firings this week may indicate a sense of alarm.

“The sheer number of 240 warplanes [involved in the joint drills] is also extraordinary,” Hong said. “Just to compare, the United States fought the early 1990s Gulf War with most of its aerial power coming from two aircraft carriers that each accommodated 80 warplanes.”

In the statement attributed to an unidentified spokesperson, North Korea’s Foreign Ministry said the United States and South Korea had created a seriously “unstable atmosphere” in the region with their military exercises. It accused the United States of mobilising its allies in a campaign using sanctions and military threats to pressure North Korea to unilaterally disarm.

“The sustained provocation is bound to be followed by sustained counteraction,” the statement said.

“[North Korea] once again clarifies that it will never tolerate any attempt of the hostile forces to infringe upon its sovereignty and security interests but respond to it with the toughest counteraction to the last,” it said.

North Korea has launched dozens of ballistic missiles this year, including multiple ICBMs and an intermediate-range missile flew over Japan. South Korean officials say there are indications North Korea in coming weeks could detonate its first nuclear test device since 2017.

Experts say North Korea is attempting to force the United States to accept it as a nuclear power and seeks to negotiate economic and security concessions from a position of strength. North Korea has punctuated its threats with an escalatory nuclear doctrine that authorises preemptive nuclear attacks in a variety of loosely defined crisis situations.

Following a meeting on Thursday at the Pentagon, Defence Secretary Lloyd Austin and South Korean Defence Minister Lee Jong-sup released a joint statement saying they “strongly condemned” North Korea’s recent weapons demonstrations.

Both defence leaders stressed that any use of nuclear weapons, including lower-yield tactical nuclear devices, against Seoul or other regional allies such as Japan, would “result in the end of the Kim Jong Un regime by an overwhelming and decisive response of the alliance,” Lee said at a joint news conference with Austin.

- Kim Tong-Hyung, AP