Secretary of State Mike Pompeo was en route to North Korea on Tuesday, President Donald Trump announced.

While the exact purpose of the trip was not immediately clear, Trump said only that Pompeo was heading there for preparations ahead of the proposed summit between Trump and North Korean leader Kim Jong Un. Congressional aides familiar with the trip told The Daily Beast that Pompeo traveled to North Korea as part of U.S. efforts to secure the release of three Americans imprisoned there.

Later Tuesday, South Korean officials said North Korea had committed to releasing the American prisoners in conjunction with Pompeo’s visit.

“At this very moment, Secretary Pompeo is on his way to North Korea in preparation for my upcoming meeting with Kim Jong Un. Plans are being made, relationships are building,” Trump said during a speech explaining his decision to withdraw the U.S. from the Iran nuclear deal.

Pompeo’s North Korea trip will be his second to the country. Last month, he traveled to Pyongyang and met with the North Korean leader to begin planning the expected meeting between Trump and Kim.

State Department spokeswoman Heather Nauert declined to elaborate on the exact purpose of the trip in a statement earlier Tuesday.

“The secretary is en route to meet with DPRK representatives. We cannot disclose the location of his meetings,” she said. “He is traveling to prepare for the historic meeting between President Trump and Kim Jong Un. We have no further announcement.”

Trump hinted at an eventual release of the American prisoners in a tweet last week, writing: “As everybody is aware, the past Administration has long been asking for three hostages to be released from a North Korean Labor camp, but to no avail. Stay tuned!”

As part of his media blitz last week, Trump attorney and former New York City Mayor Rudy Giuliani said during a Fox News interview that “we got Kim Jong Un impressed enough to be releasing three prisoners today.” That remark caused top administration officials to scramble.

Nauert sought to distance the administration from Giuliani’s comments, saying the former mayor “speaks for himself and not on behalf of the administration on foreign policy.”

Two of the prisoners held by North Korea, Tony Kim and Kim Hak-Song, were arrested during the course of Trump’s presidency.

North Korea’s state-run news outlet said Tony Kim was arrested at the airport while waiting for a flight in April 2017 “for committing criminal acts of hostility aimed to overturn the DPRK not only in the past but also during his last stay before interception.” Kim had been working in North Korea as an accounting professor at Pyongyang University of Science and Technology. The university was founded and led by James Kim, an American evangelical Christian who himself was arrested by North Korea in 1998 and charged with espionage.

Shortly after the arrest of Tony Kim, North Korean authorities detained Kim Hak Song, another Pyongyang University of Science and Technology, “on suspicion of his hostile acts against” the country as he boarded a train in Dadong bound for China, Kim’s wife told VOA. Kim became a naturalized American citizen in 2008 and had been working at the university as an agricultural researcher, helping the North to develop new technology to improve food production.

A third detainee, Kim Dong-Chul, was taken in 2015. North Korean media claimed that Kim had been “caught in the act of receiving a USB and documents with classified information of nuclear activities, military and internal reality of the DPRK” allegedly passed to him by an unnamed person in the North’s Rason Special Economic Zone. In court, Kim confessed to working with U.S. and South Korean intelligence to spy on Pyongyang—a claim widely dismissed as the result of coercion by the North—and was sentenced to 10 years of hard labor.

South Korea’s Yonhap News Agency reported last week that all three prisoners had been moved from a labor camp to a hotel near Pyongyang in apparent preparation for their release.