Pro-Russian separatists confirmed Thursday that they are holding four international observers detained in eastern Ukraine and said they would release them.

“We have taken them into custody,” Vyacheslav Ponomaryov, the self-declared people’s mayor of Sloviansk, was quoted as saying by Russia’s Interfax news agency. “We are now clarifying who they are, where they were going and why and will set them free.” The observers with the Organization for Security and Co-operation in Europe (OSCE) are being held in the Sloviansk area of the Donetsk region, the separatists said.

The OSCE said it had lost contact with its four civilian observers Monday night. The Dane, Turk, Swiss national and Estonian were on a routine patrol east of Donetsk at the time, the Danish government said.

The self-declared Donetsk People’s Republic said Wednesday that it had nothing to do with the observers’ disappearance.