A top White House official said Saturday that the odds of war breaking out with Pyongyang are moving closer to reality with each day, remarks that came just ahead of joint U.S.-South Korean air force drills — including simulated attacks on mock North Korean nuclear and missile targets.

“I think it’s increasing every day, which means that we are in a race, really, we are in a race to be able to solve this problem,” White House national security adviser H.R. McMaster told the Reagan National Defense Forum in California when asked if North Korea’s launch of an intercontinental ballistic missile Tuesday had increased the odds of conflict erupting on the Korean Peninsula.

“There are ways to address this problem short of armed conflict, but it is a race because he’s getting closer and closer, and there’s not much time left,” McMaster said in reference to North Korean leader Kim Jong Un. With each missile launch or nuclear test, Kim has seen his country’s capabilities progress, McMaster added.

North Korea, he said, represents “the greatest immediate threat to the United States.”

The nuclear-armed North test-fired a long-range missile Wednesday that it said was capable of carrying a “super-large heavy warhead.” After the launch of the Hwasong-15 ICBM, Kim declared that his country had “finally realized the great historic cause of completing the state nuclear force.”

Experts believe the missile, which likely was tipped with a dummy warhead, could have traveled up to 13,000 km (8,100 miles) — a distance that would put the whole of the continental United States within striking distance — if fired on a standard trajectory.

U.S. media, quoting senior U.S. officials, said Saturday that the missile likely broke up upon re-entry into Earth’s atmosphere.

Technical analysis of the missile flight was ongoing, but the U.S. official said “the North Koreans had problems with re-entry,” CNN reported.

Taken together with the need to master missile guidance and targeting, the re-entry failure underscores the challenges facing the country’s weapons program, according to the official.

McMaster said that U.S. President Donald Trump remains committed to denuclearizing the Korean Peninsula, while calling on Beijing to impose tighter economic sanctions on Pyongyang, noting what he called China’s “tremendous coercive economic power” over the North.

“We’re asking China not to do us or anybody else a favor,” he said. “We’re asking China to act in China’s interest, as they should, and we believe increasingly that it’s in China’s urgent interest to do more.”

China should take unilateral action to cut off North Korean oil imports, McMaster said, adding, “you can’t shoot a missile without fuel.” He said that both he and Trump felt that a 100% oil embargo would “be appropriate at this point.”

Meanwhile, the U.S. and South Korean Air Forces were on Monday set to kick off the five-day annual Vigilant Ace exercise involving 12,000 U.S. personnel and more than 230 aircraft from both countries, including U.S. stealth fighter jets from Japan, B-1B strategic bombers from Guam and advanced South Korean aircraft.

Six U.S. F-22 and F-35 stealth fighter jets each arrived in South Korea over the weekend for the exercise, the Yonhap news agency reported Sunday, citing a South Korean Defense Ministry source. Some U.S. Marine F-35Bs, stationed in Japan were also expected to take part in Vigilant Ace, it said.

The two countries will stage simulated precision airstrikes on mock North Korean nuclear and missile targets, including transporter erector launchers similar to the ones Pyongyang employs to keep its missiles mobile, the South Korean Air Force said.

Normally a rare occurrence, the dispatch to the Korean Peninsula of such advanced U.S. weaponry — known as “strategic assets” — has ramped up amid Pyongyang’s ongoing missile and nuclear tests.

North Korea has long denounced the joint military drills as a rehearsal for invasion, though Seoul and Washington have maintained that the exercises are defensive in nature.

Late Saturday, the North said Trump was “begging for nuclear war” and labeled Washington a “nuclear demon” for staging the Vigilant Ace exercise.

Late Saturday, a spokesman from the North’s Foreign Ministry blasted the “unprecedented” exercise for “simulating actual combat” scenarios, according to the state-run Korean Central News Agency.

“The U.S. is publicly touting the objective of the drill as enhancing the actual combat capability to disable the DPRK’s major strategic bases in the early stages of war,” the unidentified spokesman said.

“The Trump team is begging for nuclear war by staging an extremely dangerous nuclear gamble on the Korean peninsula,” the spokesman added.

The White House has repeatedly said that all options for reining in the North’s nuclear and missile programs remain on the table, including military action.

McMaster, however, acknowledged at Saturday’s defense forum that, given North Korea’s arsenal of conventional artillery and rockets aimed at Seoul, “there’s no military course of action that comes without risk.”

Still, he said, Pyongyang’s actions had made America’s alliances with Japan and South Korea “stronger than ever.”

Trump has variously threatened North Korea with “fire and fury” and to “totally destroy” the country of 25 million people if the United States is forced to defend itself or its allies.

The president’s incendiary language has stoked concern among some security experts and former officials that it, coupled with the joint military exercises, could lead to the U.S. stumbling into conflict.

“These are precisely the types of moves that, in normal times, could be used to bolster deterrence & reassurance,” Colin Kahl, a former deputy assistant to President Barack Obama and national security adviser to Vice President Joe Biden wrote Friday on Twitter. “But in the context of repeated threats of preventive war by Trump & McMaster could trigger miscalculation.”