(CNN) Sweden is helping negotiate the release of three Americans held captive in North Korea, sources with knowledge of the negotiations tell CNN.

American citizens Kim Hak-song, Kim Sang-duk, and Kim Dong Chul are being held by North Korean government.

Kim Sang-duk, pictured in 2011, was imprisoned in 2017.

North Korean Foreign Minister Ri Yong Ho is in Stockholm for talks with his Swedish counterpart. Sources tell CNN that Sweden "engaged heavily" in the issue of the American detainees, and since Sweden is a protective power for the US, it also represents the country's interest in all talks with North Korea.

"Any movement on the issue of these detainees would be a huge deal for the US," one source with knowledge of negotiations said, adding that Sweden did not issue any ultimatum during the weekend talks; rather, they brought it up as something that would "move things in the right direction".

Sources said the issue has been at the center of discussions for some time, both at previous low-level meetings and during North Korean Deputy Foreign Minister Han Song Ryol's visit to Sweden in January.