North Korea fired a short-range ballistic missile into the sea off its east coast on Friday, South Korea's military said, the latest in a series of launches by the nuclear-armed country.

Earlier, South Korea's military said it scrambled fighter jets when a group of about 10 North Korean military aircraft flew close to the border dividing the two countries, amid heightened tensions over repeated North Korean missile tests.

North Korea's official KCNA news agency quoted the country's military as saying it took "strong military countermeasures" after South Korean artillery-fire drills on Thursday.

The incidents followed a KCNA report on Thursday that leader Kim Jong Un had overseen the launch of two long-range strategic cruise missiles on Wednesday to confirm the reliability of nuclear-capable weapons deployed to military units.

The unprecedented frequency of North Korea's missile launches has raised concerns it may be preparing to resume testing nuclear bombs for the first time since 2017, although some analysts do not expect this before neighboring China concludes a congress of its ruling Communist Party, which begins on Oct. 16.

The U.S. Indo-Pacific Command said it was aware of the latest missile launch and had assessed that "it does not pose an immediate threat to U.S. personnel or territory, or to our allies."

"We will continue consulting closely with our allies and partners to monitor the DPRK’s destabilizing ballistic missile launches," it said, referring to North Korea by the initials of its official name.

South Korea's Joint Chiefs of Staff said the missile was launched at about 01:49 on Friday (14:49 Thursday GMT) from the Sunan area near North Korea's capital Pyongyang. It was at least the 41st ballistic missile launch by North Korea this year.

Japan's coast guard also reported that North Korea had fired what could have been a ballistic missile and that it had already fallen.

The South Korean JCS statement said the aircraft incident happened late on Thursday and early Friday Korean time.

The statement said the North Korean aircraft were detected flying about 25 km (15 miles) north of the Military Demarcation Line in the central region of the Korea border area and about 12 km (7 miles) north of the Northern Limit Line, a de facto inter-Korean border in the Yellow Sea, between 10:30 p.m. Thursday (13:30 GMT) and 12:20 a.m. on Friday (15:30 GMT Thursday).

The JCS said the aircraft were also seen near the eastern part of the inter-Korean border.

It said the South Korean air force "conducted an emergency sortie with its superior air force, including the F-35A, and maintained a response posture while carrying out a proportional response maneuver corresponding to the flight of a North Korean military aircraft."

KCNA quoted a spokesman for the General Staff of the Korean People's Army (KPA) as saying that the South Korean army had conducted about 10 hours of artillery fire near North Korea's forward defense area on Thursday.

"Taking a serious note of this provocative action by the South Korean military in the frontline area, we took strong military countermeasures," it said. "The KPA sends a stern warning to the South Korean military inciting military tension in the frontline area with reckless action."

A South Korean military spokesman said he had no information regarding the artillery fire.

North Korea has called its most recent series of missile tests, which included an intermediate-range ballistic missile that flew over Japan last week, a show of force against joint South Korean and U.S. military drills.

South Korea scrambled fighter jets a week ago after North Korean warplanes staged an apparent bombing drill as allied warships held missile defense drills in response to North Korean missile tests.

Washington imposed new sanctions last week targeting a fuel procurement network supporting Pyongyang's weapons programs.

Decades of U.S.-led sanctions have not stemmed North Korea's increasingly sophisticated weapons programs, and Kim Jong Un has shown no interest in returning to a failed path of diplomacy he pursued with former U.S. President Donald Trump.