South Korea says North Korea flew 12 warplanes near their mutual border on Thursday, prompting South Korea to launch 30 military planes in response.
South Korea's military says eight North Korean fighter jets and four bombers flew in formation. It says the North Korean planes were believed to have conducted air-to-surface firing drills.
It says South Korea responded by scrambling 30 warplanes.
Earlier, North Korea fired two ballistic missiles toward its eastern waters, and South Korea conducted naval drills with the United States and Japan off the Korean Peninsula's east coast in response.
North Korea launched two short-range ballistic missiles toward its eastern waters on Thursday after the United States redeployed an aircraft carrier near the Korean Peninsula in response to Pyongyang's previous launch of a nuclear-capable missile over Japan.
The latest missile launches suggest North Korean leader Kim Jong Un is determined to continue with weapons tests aimed at boosting his nuclear arsenal in defiance of international sanctions.
Many experts say Kim's goal is to eventually win US recognition as a legitimate nuclear state and the lifting of those sanctions, though the US and its allies have shown no sign of allowing that to happen.
The latest missiles were launched 22 minutes apart from the North's capital region and landed between the Korean Peninsula and Japan, South Korea's Joint Chiefs of Staff said in a statement.
The first missile flew 350 kilometres and reached a maximum altitude of 80 kilometres and the second flew 800 kilometres on an apogee of 60 kilometres.
The flight details were similar to Japanese assessments announced by Defence Minister Yasukazu Hamada, who confirmed that the missiles didn't reach Japan's exclusive economic zone.
He added that the second missile was possibly launched on an “irregular” trajectory. It is a term that has been used to describe the flight characteristics of a North Korean weapon modelled after Russia's Iskander missile, which travels at low altitudes and is designed to be maneuverable in flight to improve its chances of evading missile defenses.
US, South Korean and Japanese destroyers launched joint drills later on Thursday off the Korean Peninsula's east coast to horn their abilities to search, track and intercept North Korean ballistic missiles, South Korea's Joint Chiefs of Staff said.
The US destroyer is part of the strike group led by the nuclear-powered aircraft carrier USS Ronald Reagan, which returned to the waters in what South Korea's military called an attempt to demonstrate the allies' “firm will” to counter North's continued provocations and threats.
The strike group was in the area last week as part of previous drills between South Korea and the US, and the allies' other training involving Japan.
North Korea considers such US-led drills near the peninsula as an invasion rehearsal and views training involving a US carrier more provocative.
South Korea's military said it has also boosted its surveillance posture and maintains readiness in close coordination with the United States.
The US Indo Pacific Command said the launches didn't pose an immediate threat to United States or its allies, but still highlighted the “destabilising impact” of North Korea's nuclear and ballistic missile programmes.
South Korean President Yoon Suk Yeol and Japanese Prime Minister Fumio Kishida spoke by phone on Thursday and agreed that North Korea's recent missile tests are “a serious, grave provocation” that threatens international peace.
The two leaders also decided to cooperate to sternly deal with North Korean provocations, according to Yoon's office. Kishida earlier said the North's continued launches were “absolutely intolerable”.