North Korea fired off multiple short-range missiles this past weekend after denouncing Washington for going forward with joint military exercises with South Korea, according to people familiar with the situation.

The missile tests, which had not previously been reported, represent North Korean leader Kim Jong Un’s first challenge to President Biden, whose aides have not yet outlined their approach to the regime’s nuclear threat amid an ongoing review of U.S.-North Korea policy.

For weeks, U.S. defense officials warned that intelligence indicated that North Korea might carry out missile tests. The regime elevated its complaints about U.S. military exercises last week, with Kim’s sister, Kim Yo Jong, warning that if the Biden administration “wants to sleep in peace for the coming four years, it had better refrain from causing a stink.”

A senior administration official said North Korea’s move did not violate U.N. Security Council resolutions and was “a normal part of the kind of testing that North Korea would do” but underscored that the challenge of dealing with Pyongyang is significant. “We have no illusions about the difficulty this task presents to us,” the official said.

The tests put renewed pressure on the United States to develop a strategy to address a nuclear threat that has bedeviled successive Republican and Democratic administrations for decades.

A senior administration official said that the policy review is in its “final stages” and that national security adviser Jake Sullivan would host his Japanese and South Korean counterparts next week to “discuss the outcomes.”

State Department spokesman Ned Price has said the Biden administration wants to develop a “new approach” to North Korea, but he has offered few details. U.S. diplomats have informed allies in Asia in recent weeks that the strategy will differ from President Donald Trump’s top-down approach of meeting directly with Kim Jong Un and President Barack Obama’s bottom-up formulation, which swore off engagement until Pyongyang changed its behavior.

Both policies failed to stop North Korea from advancing its weapons systems and repressing its citizens through a combination of mass surveillance, torture and political-prisoner camps condemned by human rights groups around the world.

The remaining benefit of Trump’s summit diplomacy is that the regime has refrained from detonating a nuclear device or launching a long-range missile since Trump met with Kim in Singapore in 2018.

The Biden administration was mindful that it could be criticized as dithering in the event that North Korea were to restart its nuclear provocations. Those concerns became more urgent this month when U.S. intelligence detected signals that North Korea may resume its testing, said three people familiar with the situation. Satellite imagery suggesting an uptick in activity at North Korea’s Yongbyon nuclear research center published by the 38 North website also worried U.S. officials.

In an effort to inoculate the administration from potential criticism, Biden administration officials disclosed to a Reuters reporter that U.S. officials reached out to North Korea through several channels starting in mid-February but did not receive a response, said people familiar with the authorized leak. White House press secretary Jen Psaki later confirmed that attempted outreach during a press briefing.

At the time, two constituencies were pushing the administration to engage with North Korea.

Arms-control organizations based in Washington, some that have a close working rapport with the Biden administration, worried that more North Korean testing could be days away. “There is an urgent need to reengage with the North, because Pyongyang continues to amass more plutonium for nuclear weapons,” said Daryl Kimball, executive director of the Arms Control Association. “The sooner the better.”

That concern was shared by South Korea. The country’s foreign minister, Chung Eui-yong, called for an “early resumption of dialogue” between the United States and North Korea.

U.S. officials disclosed the outreach efforts to demonstrate that the administration had heard the concerns, the people said.

“The South very much wants diplomacy with the North,” said one person familiar with the matter, who like others spoke on the condition of anonymity because of the matter’s sensitivity. “They are incredibly worried that the Biden administration is going to repeat the problems of 2009 in which, for a variety of reasons, the United States was slow.”

U.S. officials did not say whether the United States made any substantive or significant proposals to North Korea in the outreach. North Korea’s first vice foreign minister, Choe Son Hui, made clear that the regime was not satisfied with what was communicated.

“We don’t think there is a need to respond to the U.S. delaying-time trick again,” Choe said in a statement carried by North Korean state media. “We will disregard such an attempt of the U.S. in the future, too.”

North Korea has not commented on its Sunday missile launches, puzzling U.S. and South Korean officials. The isolated regime typically hails such developments to underscore its technical prowess.

The State Department did not respond to a request for comment about the tests, which were discovered by U.S. officials through intelligence collection efforts outside the country.

Trump downplayed North Korea’s launch of short-range missiles during his administration, noting that they did not violate an agreement with Kim in Singapore, even though they did violate U.N. resolutions. South Korea also downplayed the moves, in the hopes of nurturing dialogue with the North.

Victor Cha, a professor at Georgetown University, said the missile test, coming after the stern warning from Kim’s sister, “was a clear challenge to the administration.” He said the North Koreans “basically have signaled . . . that they’re ready to play hardball if the Biden administration is going to play hardball.” And, he added, “they’re not done. There are more coming.”

Regardless, the Biden administration will be under more pressure to complete its policy review as dangers on the Korean Peninsula become more apparent. South Korean and Japanese officials have advised the Biden administration against reestablishing six-party talks, a multilateral framework developed during the George W. Bush administration that included China, Japan, North Korea, Russia, South Korea and the United States.

Officials from Tokyo and Seoul told their U.S. counterparts that dealing with North Korea directly would be the most productive format, advice that U.S. officials have taken seriously, according to people familiar with the discussions.

One of the challenges U.S. officials are facing in the review process is how to get countries in the region to cooperate on pressuring North Korea to denuclearize, they said.

“What is becoming clear to the architects of the new policy is how much the ground has shifted in a very short while. China is less interested in playing an active diplomatic role in the way it did during six-party talks,” said a person familiar with the discussions. “Japan and South Korea are at daggers drawn and find it difficult to even sit in the same room together, and Russia’s undermining of the American democracy has complicated any positive role with the United States in this regional endeavor.”

U.S. officials say North Korean diplomacy is also difficult because the approach must be designed so that other countries in the region support it.

“The cardinal rule of dealing with North Korea is you don’t negotiate over the heads of our allies, as Trump did when he unilaterally promised to suspend military exercises with South Korea,” said Cha, a top adviser to the Bush administration on North Korea. “You probably will get very little from North Korea. And in the bigger picture, you’re hurting our alliances in Asia.”

There are also concerns that Kim’s grip on power is less brittle than some analysts anticipated, raising questions about whether the regime can simply be coerced into giving up its weapons through punishing economic sanctions, a tactic tried by every U.S. administration.

“In spite of sanctions, North Korea has managed to build a relatively robust economy for the Pyongyang elite, quite in contrast to the deprivations that were suffered in the late 1990s because of sanctions and famine,” the person familiar with the discussions said.