(Reuters) - Pro-Russian separatist rebels in eastern Ukraine on Saturday released a second group of four monitors from the Organization for Security and Cooperation in Europe (OSCE) who had been seized on May 29, a Reuters witness said. Their release, which followed the freeing of another group of OSCE monitors early on Friday, came after Ukrainian President Petro Poroshenko announced a 72-hour extension to a ceasefire until Monday night. That ceasefire appeared under threat when three members of the Ukrainian military were killed in a rebel attack on their post near the eastern city of Slaviansk. This correspondent saw the four - three men and a woman - driven by heavily armed men up to the entrance of a hotel in the eastern city of Donetsk. They stepped out, shook hands with other waiting OSCE representatives and then went into the hotel. A first group of OSCE monitors, seized days earlier by pro-Russian separatists, were released in the early hours of Friday. The OSCE monitoring groups are part of a 300-strong force sent there to observe compliance with a four-way agreement in Geneva in April aimed at defusing the crisis in Ukraine's east. The circumstances of their detention is not yet clear. "A total of eight were detained and we have released eight," Aleksander Boroday, "prime minister" of the self-styled Donetsk People's Republic said on Friday. A ceasefire extension until Monday night announced by Poroshenko called for the release of "hostages" held by both sides. But elsewhere on Saturday the ceasefire appeared under threat when three members of the Ukrainian military were killed in a rebel attack on their post near the eastern flashpoint city of Slaviansk. "As a result of the (rebel) fighters shooting at the post near Slaviansk, three members of the Ukrainian forces were killed and a fourth was wounded," the spokesman, Oleksiy Dmitrashkovsky, was quoted as saying by the Interfax news agency. Poroshenko announced the extension, partly at the urging of some European leaders, after returning to Kiev from a European Union summit in Brussels where he signed a landmark free trade pact.
