UN Secretary-General Ban Ki-moon was "deeply concerned" about the safety of international monitors in Ukraine, some of whom were abducted earlier this week in the country.

"The UN is monitoring the situation very closely and the secretary-general is deeply concerned that international monitors who are involved in efforts to resolve the ongoing crisis in Ukraine continue to be put in grave danger, a situation that has arisen several times in recent days," Xinhua quoted Ban's spokesperson Stephane Dujarric as saying.

Four monitors of the Organisation for Security and Co-operation in Europe (OSCE) were abducted from Donetsk town in the eastern Ukraine Tuesday.

The Ukrainian Foreign Ministry said the four-member team, comprising of a Turk, an Estonian, a Dane and a Swiss, were detained by pro-Russian militants.

"The UN stresses that it is the responsibility of all involved to ensure the security of the monitors so that they may carry out their mandated functions," Dujarric said.

In April, rebels held another group of OSCE military observers for a week, mostly from Europe in Donetsk.