An American soldier who crossed into North Korea without authorization on Tuesday has been taken into custody by North Korean authorities, according to U.S. officials.

The service member entered North Korea during a tour of Panmunjom, or the Joint Security Area, which straddles the border between North and South Korea, becoming the latest U.S. citizen to be detained by the isolated Communist country.

The soldier, who was identified as Pvt. Travis T. King, had recently been released from a South Korean prison after being arrested on assault charges, according to a U.S. official familiar with the situation who was not authorized to speak publicly. U.S. military officials had planned to send him to Fort Bliss, Texas, to face additional disciplinary actions.

The service member was escorted to the airport, but instead of boarding his plane, he joined a tour of the Joint Security Area, where he broke away from the group and ran across the border, the official said, without giving further details on how the soldier was able to join the tour.

The tour guides chased after him, but did not catch him, and he was seen being taken into custody by North Korean soldiers. It was unclear if Private King planned to defect.

The soldier “willfully and without authorization crossed the Military Demarcation Line into the Democratic People’s Republic of Korea,” said Col. Isaac Taylor, a public affairs officer for the U.S. Forces Korea.

U.S. officials are working with their counterparts in the North Korean military to resolve the incident, Colonel Taylor added.

Panmunjom is a village by a collection of single-story blue-painted buildings about 30 miles north of Seoul. It was created as part of the 1953 armistice that halted the Korean War 70 years ago next week and is the sole point of contact on the approximately 150-mile-long Demilitarized Zone that separates the two Koreas. The building are overseen by the U.N. Command Military Armistice Commission, though the area is often referred to as part of Panmunjom.

The structures there were built as meeting sites for officials between the two nations, with half of a building inside North Korea and half inside South Korea. The militaries of both nations, which are still officially at war, have guards posted on their ends of the buildings and on their sides of the border.

Tour groups of foreigners are brought into the central building on both sides of the border by various tour companies, but remain on their side of the line. Tourists often have photos taken with guards standing in the background.

Outside the buildings, a concrete strip on the ground demarcates the border. In June 2019, President Donald J. Trump stepped across the border at Panmunjom and walked 20 paces to the base of a building in North Korea to greet Kim Jong-un, the country’s leader. Mr. Trump was the first American president to set foot in North Korea.

The United States and North Korea do not have formal diplomatic relations, and U.S. interests in the country are represented by the Swedish Embassy there.

Matthew Miller, a State Department spokesman, said that the department had not been in touch with North Korea, but that the Pentagon had tried to make contact. “It is our understanding that the Pentagon has reached out to their counterparts” in North Korea, he said. “They’re the lead agency, and I will defer to them to comment on the nature of those contacts.”

During a news conference on Tuesday, Defense Secretary Lloyd J. Austin III said the U.S. military was “closely monitoring and investigating the situation, and working to notify the soldier’s next of kin.”

“I’m absolutely foremost concerned about the welfare of our troop,” Mr. Austin said.

The American-led United Nations Command first announced the border crossing on Tuesday in an online post. Both the U.N. Command and the North Korean People’s Army keep duty officers at Panmunjom.

The soldier was the first known American held in North Korean custody since​ Bruce Byron Lowrance​ was detained for a month after illegally entering the country from China in 2018.​

The American student Otto F. Warmbier was arrested in Pyongyang, North Korea’s capital, in 2016, accused of trying to steal a propaganda poster from the wall of his hotel. He was sentenced to 15 years in prison. After being held for 17 months in North Korea, Mr. Warmbier, then 20, was flown to Ohio, his home state, in a coma in June 2017. He died a week later.

​Although the border area is strewn with land mines and guarded by layers of tall barbed-wire fences, people from both Koreas have crossed the DMZ, as have several American soldiers stationed in the South.

In 2014, an unidentified American was detained on a riverbank near the South’s western border with North Korea after trying to swim into the North. After he was apprehended, he told South Korean officials that he had intended to go to North Korea to meet Mr. Kim. Before he entered North Korea from China, Mr. Lowrance was also detained by South Korean soldiers while approaching the inter-Korean border.

But defections through Panmunjom are highly unusual.

A South Korean soldier assigned to the Joint Security Area defected to the North in 1991. In 2017, a North Korean soldier​ ran across ​Panmunjom through a hail of bullets ​from fellow Communist soldiers trying to stop him. The defector survived multiple bullet wounds.

Relations between North Korea and the United States have deteriorated in recent years as the North has ramped up its nuclear and missile programs, defying international sanctions.

Early Wednesday on the Korean Peninsula, North Korea fired two short-range ballistic missiles off its east coast, according to the South Korean military. ​The North Korean government had expressed anger ​for days over the Pentagon’s plan to send to South Korea a nuclear-armed submarine. North Korea fired its newest intercontinental ballistic missile, the Hwasong-18, last Wednesday.

The fate of American citizens held in North Korea is not always clear. Some are voluntarily released, while others have faced criminal charges of committing “hostile acts” and​ have been freed only when American officials, like former President Bill Clinton, have visited Pyongyang to request their release.

North Korea released three American detainees in ​2018 after Secretary of State Mike Pompeo visited Pyongyang to pick them up. North Korea treated their release as a sign of good will and a merciful diplomatic gesture aimed at facilitating ​Mr. Kim’s summit meeting with Mr. Trump in Singapore ​later that year.

Choe Sang-Hun reported from Seoul, and Michael D. Shear, John Ismay and Edward Wong reported from Washington.