North Korean leader Kim Jong Un said his country’s test of a “new-type” of guided missile Tuesday was a warning over joint U.S.-South Korea military exercises, state media said Wednesday, amid fears of a possible return to 2017’s ramped-up pace of weapons testing and threats of “fire and fury.”

The North’s official Korean Central News Agency said in a report that Kim had overseen the test-firing of the “tactical guided missiles” from an airfield in the country’s west that saw the weapons fly across the country and over the capital, Pyongyang, “to precisely hit” an island target in the Sea of Japan.

“The demonstration fire clearly verified the reliability, security and actual war capacity of the new-type tactical guided weapon system,” KCNA reported.

It said Kim had noted that the “military action would be an occasion to send an adequate warning to the joint military drill now underway by the U.S. and south Korean authorities.”

On Tuesday, North Korea criticized the military exercises after launching the two apparent short-range ballistic missiles, conducting its fourth weapons test in less than two weeks.

The South Korean military said the weapons appeared to be short-range ballistic missiles that resembled those fired July 25 and traveled around 450 km at around Mach 6.9 and hit an apogee of around 37 km.

Tuesday’s missiles came after North Korea lobbed two short-range weapons into the Sea of Japan on Friday and sent two other short-range ballistic missiles into the waters a week ago. On July 25, Pyongyang also fired two rounds of a “new type” of ballistic missile, resuming its weapons testing after 77 days.

U.S. national security adviser John Bolton said in an interview with Fox News Channel on Tuesday that the testing appeared aimed at getting the short-range missiles fully operational and that U.S. President Donald Trump was keeping a close eye on developments.

“The president and Kim Jong Un have an understanding that Kim Jong Un is not going to launch longer-range, intercontinental-range ballistic missiles, and so I think the president is watching this very, very carefully,” he said.

The ramped-up pace of testing has stoked fears of a return to 2017, when the nuclear-armed country conducted 21 missile tests, including launches of intermediate-range ballistic weapons over Japan and an intercontinental ballistic missile capable of striking most, if not all, of the contiguous United States.

Experts, however, have said that the increased clip of missile testing is merely the latest example of Kim taking a page from the well-worn North Korean negotiating playbook.

“North Korea is, as ever, raising tensions in a calculated way within certain bounds,” said Mintaro Oba, a former U.S. State Department official who worked on North Korean issues.

He said the tests and warnings against the scaled-down U.S.-South Korean joint military exercises that kicked off Monday were likely Kim “making clear that the … tests are a proportional, situational response” to the drills while also maintaining caution ahead of a restart of stalled denuclearization talks with Washington.

“So, while North Korea may continue to use tests as a means of pressuring the United States and South Korea, perhaps even increasing the pace and range a bit, Pyongyang will most likely avoid escalating so much that it kills the diplomatic process,” Oba added.

On Tuesday, the North’s Foreign Ministry ripped into the military exercises, labeling them a “flagrant violation” of efforts to reach peace on the Korean Peninsula and reflecting a lack of “political will” by Seoul and Washington to improve relations. The spokesman said that if the military exercises continued, the North “will be compelled to seek a new road as we have already indicated.”

U.S. officials have said stalled denuclearization talks with the North Koreans will happen “soon,” but have admitted that a time and location have yet to be set.

The U.S. said after Trump’s meeting with Kim at the truce village of Panmunjom on the border between the two Koreas in late June that the talks would begin sometime in July. That time frame has come and gone, but the U.S. has said the two sides were still in contact.

The military drills are reportedly command-post exercises mostly involving computer simulations, not mobilization of troops or military equipment, lasting about two weeks, while also testing South Korea’s capabilities in retaking operational control over its forces from the United States during wartime.

The exercises, which began on a low-key note, went ahead despite earlier warnings by Pyongyang, which views the drills as a rehearsal for invasion.

A U.S. Forces Korea spokesman told The Japan Times earlier this week that the two militaries “continue to train in a combined manner at echelon while harmonizing our training program with diplomatic efforts by adjusting four dials: size, scope, volume and timing.”

Observers say Trump is betting that by keeping the joint exercises low-key, the U.S. can still convince the North to return to the bargaining table once the drills wrap up.

But David Kim, a research analyst with the Stimson Center think tank in Washington, said that the exercises could potentially “dampen the spirit of future working-level negotiations, on top of an already shaky relationship that hasn’t seen much traction since” the second Trump-Kim summit in Hanoi at the end of February. Those talks collapsed amid major differences over the scope of Pyongyang’s denuclearization and potential sanctions relief by Washington.

“For now, I don’t expect negotiations to resume as long as the U.S.-South Korea drills continue … so I think it will be a while before we see traction on talks,” Kim, a former U.S. State Department nonproliferation and East Asia desk official, said. “I think Trump will need to inject fresh energy into the diplomatic process at some point before the end of the year.”