UNITED NATIONS, May 30 (Xinhua) -- UN Secretary-General Ban Ki- moon was "deeply concerned" about the safety of international monitors in crisis-torn Ukraine, some of whom were reportedly abducted earlier this week in the restive eastern part of the country, a UN spokesman said here on Friday.

"The United Nations is not directly involved in obtaining their release," said Stephane Dujarric, Ban's spokesperson, at a daily briefing when he was asked about reports that the Organization for Security and Co-operation in Europe (OSCE) had not been able to establish contact with its monitors in Ukraine.

"The UN is monitoring the situation very closely and the secretary-general is deeply concerned that international monitors who are involved in international 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," Dujarric said.

He noted that the abduction is to be deplored and anyone who has influence should use it to press for their release.

"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 added.

On the Ukraine situation, the spokesperson said that the UN chief was very distressed by reports of violence and reiterated his call for dialogue as the only means for a peaceful solution to the crisis. "He reminds all parties with influence to exercise utmost restraint."

The OSCE said Tuesday it had lost contact with the four-person monitoring team, which was on an observing mission near Ukraine's Donetsk.

The Ukrainian Foreign Ministry said the team, comprising a Turk, an Estonian, a Dane and a Swiss, was detained by activists.

However, Interfax news agency quoted the self-styled leader of the "People's Front" in Lugansk region Alexei Chmilenko as saying on Friday that four OSCE observers that were "detained a few days ago and taken to Lugansk for document checks" have been released after "it was confirmed that they are indeed representatives of the international organization, not involved in provocation."

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