Tension between the United States and North Korea escalated anew Thursday as Pyongyang conducted a fresh round of missile tests and the Trump administration announced the seizure of a North Korean ship accused of smuggling coal and heavy machinery in defiance of international sanctions.

National security sources said the two developments underscored the widening shadow hanging over the future of denuclearization talks with North Korea. The talks have been stalled since late February, when President Trump’s second high-stakes summit with North Korean leader Kim Jong-un ended without a deal.

Officials said the North Korean cargo ship in question was seized in April 2018 by Indonesian authorities, but the announcement Thursday that the vessel is now in U.S. custody revealed a first-of-its-kind sanctions enforcement against Pyongyang by the Trump administration.

The Justice Department made the announcement just hours after military leaders in South Korea confirmed that the North had fired two short-range missiles from the city of Kusong. Unease over the launches, which marked Pyongyang’s second weapons tests in five days, was elevated later Thursday by reports that the North also conducted long-range strike dills during the day.

Analysts said the moves were particularly provocative given that Stephen E. Biegun, U.S. special representative for North Korea, was in Seoul at the time of the short-range test launches.

South Korean President Moon Jae-in quickly condemned the tests but said they stemmed from pure frustration in the North over the failure of February’s summit between Mr. Trump and Mr. Kim in Hanoi, Vietnam. The summit was cut short after the two failed to strike a far-reaching deal to end the North’s nuclear and missile programs. Mr. Trump said he had to walk away because Mr. Kim demanded sweeping sanctions relief in exchange for only a limited commitment to destroy part of his nuclear arsenal. Pyongyang later challenged that characterization.

Mr. Moon, who has played an essential role in facilitating dialogue between Washington and Pyongyang, said the Kim regime is publicly lashing out in the hope of forcing Washington to offer relief from economic sanctions. “It is protesting to the United States and South Korea,” the South Korean president said, according to reports by the Yonhap News Agency in Seoul.

Mr. Trump has insisted that North Korea commit to full denuclearization before any sanctions are lifted, but administration officials since Hanoi have also embraced a tempered posture toward North Korean provocations and rhetoric.

The administration’s response to Thursday’s missile tests was mostly muted, although analysts said the president’s foreign and national security advisers may be weighing a response while their focus is consumed by other challenges, including Iran’s most recent threats to restart its nuclear program and continued political upheaval in Venezuela.

Mr. Trump briefly addressed the tests but, as was the case after launches last week, he stopped short of delving into the sort of “fire and fury” rhetoric that he used in 2017 when North Korean nuclear bomb and intercontinental ballistic missile tests pushed tensions near a breaking point.

“Nobody’s happy about it,” Mr. Trump said of Thursday’s tests before reiterating that North Korea has “tremendous economic potential” it can unlock through a denuclearization deal with the United States.

The Justice Department, however, took direct aim at North Korea by delivering a clear message that the U.S. will vigorously enforce its economic sanctions.

For the first time, federal authorities revealed how they have taken possession of a massive North Korean ship for violations of international sanctions.

Prosecutors said the 17,601-ton cargo vessel, dubbed the “Wise Honest,” is the second largest in North Korea’s fleet. They say Pyongyang used the ship to skirt sanctions imposed by the U.S. and the United Nations to export coal, a critical part of the isolated nation’s economy.

“This scheme not only allowed North Korea to evade sanctions, but the Wise Honest was also used to import heavy machinery to North Korea, helping expand North Korea’s capabilities and continuing the cycle of sanctions evasion,” said Geoffrey Berman, U.S. attorney for the Southern District of New York.

Assistant Attorney General John C. Demers said, “This sanctions-busting ship is now out of service.”

In a request filed in July, Justice Department prosecutors asked a New York federal judge to give the U.S. ownership of the vessel through a civil forfeiture action. It is the same action prosecutors take when they seize boats or cars used by drug dealers.

The request was unsealed Thursday.

Prosecutors said they have a right to take the ship because American banks unwittingly serviced payments for maintenance, equipment and improvements of the Wise Honest, violating international sanctions.

Analysts said the ship seizure, coupled with the missile tests, shows how quickly the diplomatic goodwill that seemed to be developing between Washington and Pyongyang prior to the Hanoi summit could evaporate.

“We’re at an impasse,” said Bruce Klingner, a senior research fellow at The Heritage Foundation and a former CIA Korea deputy division chief.

“Right now, both sides think that the ball is in the other’s court to take action,” he said. “The U.S. is not reacting strongly to these [weapons] tests, and if North Korea doesn’t get satisfaction, it may decide it’s necessary to do something.”

If Mr. Kim doesn’t get what he wants from Washington, then he could restart nuclear tests in a last-ditch attempt to force the administration to make concessions, Mr. Klingner said.

“As North Korea gets frustrated, it may continue going down that path,” he said, describing nuclear bomb tests as a potential “red line” for Mr. Trump — a point at which the president may conclude he has no choice but to resort to military action against North Korea.

Others say the very notion of such a scenario makes it all the more vital for negotiations to continue.

“All efforts should be made to revive diplomacy and find a way back to the bargaining table, as another round of ‘fire and fury’ and nuclear threats might not end peacefully this time around,” said Harry J. Kazianis, director of Korean Studies at the Center for the National Interest.

Critics of the president have seized on the missile tests to cast Mr. Trump as woefully naive in his continued willingness to negotiate with Mr. Kim.

“The launch of short-range missiles by North Korea is yet another sign that President Trump is in over his head with respect to North Korea,” said Rep. Ruben Gallego, Arizona Democrat. “The president set himself and our country up for failure with his unilateral meeting with Kim Jong-un last year, when he should have been working with our allies, especially South Korea, to reduce tensions and move toward a deal that is in our national interest.”

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