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 missiles tests.

A statement on North Korea’s official KCNA news agency quoted the North Korean military as saying it took “strong military countermeasures” after South Korean artillery-fire drills on Thursday.

South Korea’s Joint Chiefs of Staff said the missile was launched at about 01:49 on Friday (1449 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.

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 (1330 GMT) and 0:20 a.m. on Friday (1530 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 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,” it added.

A spokesman for South Korea’s military said he had no information regarding the South Korean artillery fire reported by the North.

North Korea has said that its most recent series of missile tests, which included an intermediate range ballistic missile that flew over Japan last week, was 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.