Foreign Minister Urmas Paet has called for the immediate release of four observers, including one Estonian, who were went missing in the Donetsk area and were later reported taken prisoner by separatists, with the latest news from the area suggesting the men may be released soon.
Fresh reports from the BBC say pro-Russian separatists have admitted to holding the four men, adding that they may soon release the team.
"We'll clarify who they are, where they were heading and why, and will set them free," a self-proclaimed mayor of Sloviansk told news Russian agency Interfax.
BBC reported on Tuesday that the last contact with the group was on Monday evening. The Estonian Ministry of Foreign Affairs confirmed the report.
"This is completely unacceptable that the activities of the OSCE observers are impeded in this way," Paet said on ETV on Wednesday.
The name of the Estonian was not released by the Foreign Ministry under an agreement with the OSCE. The OSCE said the team was part of a special monitoring mission and on a routine patrol east of Donetsk on Monday evening, when contact was lost.
One member each from Turkey, Switzerland and Denmark are also in the group from the Organization for Security and Co-operation in Europe.
Seven OSCE observers were detained by separatists at the end of April, and released a week later.
This story will be updated as events unfold.