Mike Pompeo is expected to secure the release of the three American detainees held in the country, a South Korean official said, according to Reuters.

U.S. officials said the move would be significant goodwill gesture ahead of the President Trump-Kim Jong Un summit expected later this month or in early June. Pompeo, who arrived in North Korea on Tuesday, said it would be difficult to hold a leaders’ summit between Trump and Kim if the prisoners remained captive.

The three U.S. detainees still being held are Korean-Americans Kim Dong Chul, Kim Hak Song, and Tony Kim, who also uses the name Kim Sang Duk.

Dong Chul, a former Virginia resident, has been held the longest, sentenced in April to 10 years in prison with hard labor after being convicted of espionage.

Hak Song worked in agricultural development at an experimental farm run by the Pyongyang University of Science and Technology (PUST), which was founded in 2010 with donations from Christian groups. He was detained last May for alleged anti-state activities and accused of “hostile acts,” Fox News reported.

Sang Duk taught accounting also at PUST and was detained a year ago at the Pyongyang Airport. He was accused of committing unspecified “hostile” criminal acts intended to overthrow the North Korean government.

The only American released by North Korea during Trump’s presidency has been the late Otto Warmbier, 22, who returned to the U.S. in a coma last summer after 17 months of captivity, Reuters reported. Warmbier died days after his return in June 2017, which escalated U.S.-North Korea tensions, the report said.

On Tuesday, Kim met with Chinese President Xi Jinping in China, the second trip to China in less than two months. Trump and Xi discussed Kim’s visit to China in a phone call Tuesday morning, the White House said, according to Reuters.

The Associated Press contributed to this report.