SEOUL, South Korea >> North Korea fired two short-range ballistic missiles and artillery shells toward its eastern waters Monday, escalating animosities over U.S.-South Korean military drills that it views as an invasion rehearsal.

The weapons firings follow an intercontinental ballistic missile launch Saturday and North Korea’s threats to take an unprecedented strong response to the drills. A new testing spree also allows North Korea to expand its arsenals amid stalled talks with its rivals and eventually use the boosted military capability as leverage to wrest bigger concessions from the United States.

South Korea detected the two missile launches from a western coastal town, just north of Pyongyang, the North Korean capital, on Monday morning, South Korea’s Joint Chiefs of Staff said in a statement.

It said South Korea has boosted its surveillance posture and maintains a readiness in close coordination with the United States.

Japan’s Defense Ministry said both missiles landed in the waters between the Korean Peninsula and Japan. It said Japan condemned the launches as a threat to the peace and safety of Japan and the international society.

The Japanese Defense Ministry said the first missile reached the maximum altitude of 100 kilometers (62 miles) and flew as far as 400 kilometers (250 miles). It said the second missile reached about 50 kilometers (30 miles) in altitude and flew a distance of 350 kilometers (217 miles).

Also Monday morning, North Korea’s long-range artillery units on its western coast fired two rounds cross-country toward the eastern waters in response to what its state media called the U.S.-South Korean aerial drills on Sunday, according to the North’s official Korean Central News Agency. KCNA said the North Korean artillery rounds simulated strikes on targets up to 395 kilometers (245 miles) away.

The North said the launches involved its new 600 millimeter multiple rocket launcher system that could be armed with “tactical” nuclear weapons intended for battlefield use. Some experts viewed the weapons system as a short-range ballistic missile.

“The frequency of using the Pacific as our firing range depends upon the U.S. forces’ action character,” Kim Yo Jong, the powerful sister of North Korean leader Kim Jong Un, said in a statement carried by state media. “We are well aware of the movement of U.S. forces’ strategic strike means recently getting brisk around the Korean Peninsula.”

Calling the United States “the worst maniacs,” she threatened to take unspecified “corresponding counteraction” in response to the future moves by the U.S. military.

She could be referring to the U.S. flyover of B-1B long-range, supersonic bombers on Sunday for separate training with South Korea and Japan. The B-1B deployment came as response to North Korea’s launch of the Hwasong-15 ICBM off its east coast on Saturday in the country’s first missile test since Jan. 1.

North Korea is extremely sensitive to the deployment of B-1B bombers, which can carry a huge payload of conventional weapons.

North Korea’s state media said Sunday the ICBM test was meant to further bolster its “fatal” nuclear attack capacity and verify the weapon’s reliability and the combat readiness of the country’s nuclear force. In her earlier statement Sunday, Kim Yo Jong threatened to take additional powerful steps over upcoming military drills between the United States and South Korea.

North Korea has steadfastly slammed regular South Korea-U.S. military drills as a practice for a northward invasion though the allies say their exercises are defensive in nature. Some observers say North Korea often uses its rivals’ drills as a pretext to hone and perfect its weapons systems.

The South Korean and U.S. militaries plan to hold a table-top exercise this week to hone a joint response to a potential use of nuclear weapons by North Korea. The allies are also to conduct another joint computer simulated exercise and field training in March.

Last year, North Korea set an annual record with the launch of more than 70 missiles. North Korea has said many of those weapons tests were a warning over previous U.S.-South Korean military drills. It also passed a law that allows it to use nuclear weapons preemptively in a broad range of scenarios.

Kim Jong Un entered 2023 with a call for an “exponential increase” of the country’s nuclear warheads, mass production of battlefield tactical nuclear weapons targeting South Korea and the development of more advanced ICBMs targeting the U.S.