Test ramps up the stakes for U.S. President Donald Trump, who has vowed to rein in North Korea’s arms program

SEOUL—North Korea test-fired a ballistic missile Friday that experts said put the continental U.S. firmly in range of a strike, underscoring Pyongyang’s rapid advance in technology and intensifying a standoff with Washington.

The launch emanated from North Korea’s mountainous interior, flying for more than 45 minutes before landing in the waters between Japan and the Korean Peninsula, U.S., South Korean and Japanese officials said.

Hours after the North Korea missile launch, the U.S. and South Korea carried out a live fire exercise by launching missiles into the territorial waters off South Korea’s East Coast, according to the U.S. military.

Lt. Gen. Thomas Vandal, commander of the U.S. Eighth Army in South Korea, said that the U.S. military is “ready to fight tonight, will deter North Korean provocation and if necessary defend the Republic of Korea,” using the formal name for South Korea.

North Korea’s action comes just three weeks after it surprised the world with its first ever intercontinental ballistic missile test.

But Friday’s missile was a more advanced ICBM than the one fired earlier this month, flying 620 miles and reaching a maximum altitude of 2,300 miles—far more than the July 4 missile’s 1,740 miles, South Korea’s Joint Chiefs of Staff said. North Korea, in a separate media report hours later, released numbers in line with those estimates.

The significantly higher altitude of Friday’s missile suggests that it could have flown much farther than the last one.

The new missile would be able to fly more than 6,400 miles if fired at a standard trajectory, David Wright, a physicist and co-director of the Global Security Program at the Union Concerned Scientists, wrote in an analysis on Friday. That would put Los Angeles, Denver and Chicago within range.

In a report published Saturday morning by the official state mouthpiece, North Korean leader Kim Jong Un said the ICBM test showed Pyongyang could make a “surprise launch of ICBM in any region and place any time, and clearly proved that the whole U.S. mainland is in the firing range of the DPRK missiles.” He used the acronym for North Korea’s formal name, the Democratic People’s Republic of Korea.

“The powerful war deterrence for defending the state is an inevitable strategic option and it is a precious strategic asset that cannot be bartered for anything,” Mr. Kim was quoted as saying. “If the Yankees brandish the nuclear stick on this land again despite our repeated warnings, we will clearly teach them manners with the nuclear strategic force.”

The test ramps up the stakes for U.S. President Donald Trump, who has vowed to halt North Korea’s weapons program as a top foreign policy priority.

The White House denounced the test as another “reckless and dangerous” provocation. “The United States will take all necessary steps to ensure the security of the American homeland and protect our allies in the region.”

The launch also comes as experts say North Korea is advancing faster than expected on another vital component of a long-range nuclear missile: its ability to miniaturize a nuclear device on a missile warhead that can handle atmospheric re-entry.

House Armed Services Committee Chairman Mac Thornberry (R., Texas) said that makes the need to address North Korea’s threat more urgent.

“It always should be a wake-up call when you realize you were wrong on previous estimates,” Mr. Thornberry told The Wall Street Journal. “It ought to be a sobering wake-up call and it ought to have some action attached to it.”

North Korea also test-fired the missile from a remote part of the country from which it has never done so before, according to data from the Nuclear Threat Initiative in Washington, a nonprofit advocacy group, highlighting Pyongyang’s ability to defy expectations about where and when it can launch a strike.

South Korean President Moon Jae-in said early Saturday that Seoul would consider deploying more components of a U.S. missile-defense system in South Korea in response to the latest test-launch.

China objects strenuously to the U.S.-made missile-defense system—the Terminal High Altitude Area Defense—saying it compromises Beijing’s national security.

Gen. Joe Dunford, chairman of the Joint Chiefs of Staff, and Adm. Harry Harris, commander of the U.S. Pacific Command, gathered in the Pentagon to call Gen. Lee Sun Jin, chairman of South Korea’s Joint Chiefs of Staff, and discussed how to respond.

In Tokyo, Japanese Prime Minister Shinzo Abe denounced the launch and said Tokyo would work with the U.S. to strengthen pressure on Pyongyang.

The missile firing occurred a day after the 64th anniversary of the end of the Korean War, a politically significant holiday for North Korea, when several missile experts were closely watching for such an event. The late timing may have been dictated by rain and cloudy weather that covered much of the country on Thursday and Friday.

North Korean state media said the launch affirmed the country’s aim to reach “the U.S. heartland” with an ICBM, referring to a “package of gifts” to Washington.

The missile’s relatively long flight time, combined with the relatively modest lateral distance traveled, suggests that the missile was fired at an unusually steep trajectory.

Firing at such a steep trajectory allows Pyongyang to test longer-range missiles without sending it over any of its neighbors, including Japan, China and Russia, which would risk provoking a military response.

The previous test-launched missile, which North Korea called the Hwasong-14, or the Mars-14, was deemed by experts to be capable of reaching Alaska if fired on a less steep trajectory.

Russia, however, disputed the claim that the July 4 missile was an ICBM, generally defined as a missile that can fly farther than 5,500 kilometers.

Catherine Dill, a Washington-based senior research associate for the Middlebury Institute, said in an interview that any doubts about whether North Korea’s previous launch was an ICBM would be dispelled by Friday’s launch.

“It seems that this test was a direct response to” North Korea’s skeptics, Ms. Dill said. If Friday’s missile was the same model as the one fired earlier this month, Pyongyang would likely have enough confidence to begin putting the missile into production for deployment.

“If this was indeed the same missile, they’ll have two successful tests, and in the past, after two successful tests they start to begin fielding them,” she said. On North Korea’s ability to miniaturize a nuclear device, she said, “If they’re able to master this level of technology, I don’t see why they can’t master miniaturization,” she said.

North Korea in its state media said the missile was the same Hwasong-14 model as the one it launched on July 4, except conducted to “its maximum range.” Like the July 4 launch, the Friday missile was “capable of carrying large-sized heavy nuclear warhead,” Pyongyang said, adding that the launch was conducted at the maximum steep trajectory to avoid impacting neighboring countries.

North Korea also claimed success in the ability of its warhead to withstand atmospheric re-entry, saying “the warhead explosion control device showed normal operation even at the thousands of degrees of Centigrade.”

Mr. Kim said the test-firing “proved to be a perfect and big success without an inch of error.”

After Friday’s launch, U.S. Secretary of State Rex Tillerson urged world leaders to tighten United Nations sanctions on North Korea and singled out Russia and China for not doing enough to contain Pyongyang’s nuclear ambitions.

“As the principal economic enablers of North Korea’s nuclear-weapon and ballistic-missile development program, China and Russia bear unique and special responsibility for this growing threat to regional and global stability,” he said.

​A spokesman for China’s Foreign Ministry said it was opposed to any further North Korean missile launches and urged all sides to “act cautiously” ​to avoid further inflaming tensions on the peninsula.