Estonian Foreign Minister Urmas Paet called for the immediate release of four OSCE observers, including one Estonian, who were went missing in the Donetsk area in Eastern Ukrain, Estonian Public Broadcasting (ERR) reported on Tuesday evening.

An earlier BBC report said the last contact with the group was on Monday evening, and the group of four observers was taken prisoner by armed separatists. The news was later confirmed by the Estonian Ministry of Foreign Affairs.

"This is completely unacceptable that the activities of the OSCE observers are impeded in this way," said Paet on ERR's ETV Tuesday evening.

However, the name of the Estonian among the four detainees was not released by the Foreign Ministry under an agreement with the OSCE, the Organization for Security and Cooperation in Europe.

The OSCE said the team was part of a Special Monitoring Mission and on a routine patrol in the east of Donetsk on Monday evening, when contact was lost. The other three members are each from Turkey, Switzerland and Denmark.

The four observers disappearance came after another seven OSCE observers were detained by separatists in the restive Ukraine at the end of April and were released a week later, the ERR report added.