North Korea fired two short-range missiles into the sea between the Korean peninsula and Japan on Monday morning, registering its displeasure with the start of annual military exercises between South Korea and the United States.

After offering to suspend nuclear tests if the United States and South Korea canceled the drills, Pyongyang has reverted saber rattling, threatening “merciless strikes” just hours before the missiles were launched.

North Korea fired the two missiles, thought to be Scud-C or Scud-D types, from the western coastal city of Nampo, about 300 miles over the peninsula and into the sea off its east coast, South Korea’s Joint Chiefs of Staff said Monday. The launches took place at 6:32 a.m. and 6:41 a.m., it said in a statement.

They were launched as South Korea and the United States began military exercises to practice to coordinate their response to the North Korean threat: The two-week-long computer-simulated Key Resolve drill, and the Foal Eagle field exercises, which will continue through April 24.

North Korea has protested the drills year after year, calling them a “dress rehearsal” for an invasion.

“Key Resolve and Foal Eagle are an undisguised encroachment upon the sovereignty and dignity of the DPRK and an unpardonable war hysteria of dishonest hostile forces,” the general staff of the North Korean People’s Army said in a statement issued before the launches Monday.

“Our revolutionary armed forces will never remain a passive onlooker to this grave situation,” it said, vowing “merciless strikes” against such aggression.

South Korea and Japan sharply criticized North Korea’s actions.

“Its menacing words and deeds are a very grave challenge to the security of the Korean peninsula and firing ballistic missiles is in violation of the United Nations’ resolutions,” Kim Min-seok, a spokesman for the South’s defense ministry, told reporters in Seoul.

“Our military squarely warns the North against its reckless provocations and will respond in a stern and strong manner based upon a staunch combined defense posture,” he said, according to the Yonhap News Agency.

Shinzo Abe’s government in Tokyo also said it would lodge a strong protest with North Korea.

Earlier this year, North Korea said it was willing to suspend nuclear tests if the United States and South Korea canceled the annual military drills, but the State Department dismissed the offer, calling it “an implicit threat.”