WASHINGTON (Reuters) - The latest detention of a U.S. citizen by North Korea, which raises the number of Americans in their custody to four, is “concerning,” a White House spokesman said, amid heightened tensions over Pyongyang’s nuclear and missile programs.

FILE PHOTO - A North Korean flag flies on a mast at the Permanent Mission of North Korea in Geneva October 2, 2014. REUTERS/Denis Balibouse/File Photo

“Obviously, this is concerning. We’re well aware of it and we’re going to work through the Embassy of Sweden ... through our State Department, to seek the release of the individuals there,” White House spokesman Sean Spicer told reporters on Monday.

Kim Hak Song, who worked for the Pyongyang University of Science and Technology, was detained on Saturday on suspicion of “hostile acts” against the state, North Korea’s official KCNA news agency said.

Four U.S. citizens are currently being held in North Korea, a country with which the United States has no diplomatic relations and has technically remained at war with since the 1950-53 Korean conflict.

An official of the U.S. State Department said the detentions underscored “the risk associated with travel to North Korea.”

“The Department of State strongly recommends against all travel by U.S. citizens to North Korea,” the official added.

Japanese media earlier on Monday reported that Choe Son Hui, the director general of the North Korean Foreign Ministry’s North America section, was traveling to Europe for unofficial talks with former U.S. government officials.

The so-called track two talks would be the first since U.S. President Donald Trump took office in January and come at a time of tensions over North Korea’s efforts to develop a nuclear missile that can hit the United States.

A similar round of talks, also involving Choe, had been due to take place in the United States in March, but the North Korean delegation was denied U.S. visas at the last moment after the killing of the half-brother of North Korea’s leader Kim Jong Un at the international airport in Kuala Lumpur, Malaysia, an assassination U.S. officials blamed squarely on Pyongyang.

A source with knowledge of the latest talks said the venue was the Norwegian capital, Oslo, and the U.S. delegation was led by Suzanne DiMaggio, a director and senior fellow at the New America think tank.

While at least one former U.S. government official was participating, the U.S. administration was not involved, the source added, speaking on condition of anonymity.

Contacted by Reuters, DiMaggio, an Iran specialist, confirmed she was currently in Oslo, but declined to give details of the talks.

A State Department official played down the significance of the meeting, saying: “Track two meetings are routinely held on a variety of topics around the world and occur independent of U.S. government involvement.”

Trump warned in an interview with Reuters this month a “major, major conflict” with North Korea was possible, but then raised eyebrows by saying he would be “honored” to meet North Korean leader Kim Jong Un under the right circumstances.

U.S. officials say North Korea must agree to give up its weapons programs if it wants formal talks to resume.