Three members of the Ukrainian military were killed on Saturday when pro-Russian separatists attacked their post near the eastern flashpoint city of Slaviansk with small arms and mortar fire, a military spokesman said.
The reported attack on the post manned by members of the government's "anti-terrorist operation" took place despite a government ceasefire, which was extended on Friday until Monday night.
"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.
President Petro Poroshenko announced the extension on Friday night, 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.
Also on Saturday, Ukrainian separatists refused to surrender the border checkpoints under their control to the Ukrainian army.
At a news conference in eastern Ukraine one of the leaders of the self-proclaimed Luhansk Republic Alexei Karyakin told journalists that three border checkpoints, that Kiev authorities wanted back as a condition of the ceasefire, were used to allow civilians to escape the conflict zone.
"Ukrainian authorities demanded from us to free border checkpoints of Izvarino, Krasnyh Partizan and Dolzhansky, so that the Ukrainian armed forces would enter those checkpoints. On the issue our position remains unchangeable and we have said that the checkpoints would stay under our control as they are part of our territory," Alexei Karyakin said.
"We agreed that OSCE representatives would stay at those checkpoints and monitor humanitarian corridors that we provide for for the refugees to cross onto the territory of the Russian Federation," he added.
In another rebel stronghold of Donetsk the pro-Russian separatists captured an Interior Ministry base with over hundred troops on Friday (June 27) night. The troops were said to have surrendered without fighting back.
On Saturday one of the leaders of self-proclaimed Donetsk People's Republic Vladimir Makovich offered Kiev controlled troops either to join the rebels or be dismissed.
"According to the order of Commander-in-Chief, Defence Minister of Donetsk People's Republic, Igor Ivanovich Strelkov, you will be dismissed," said Vladimir Makovich.
"You have the right to choose - either go home or somewhere else or stay here and continue your service. The choice is yours," he added.
Ukrainian base commander Colonel Oleh Ponomarenko was applauded by his troops when he thanked his soldiers for their service under his command.
Troops that opted to go home stayed in the conference room and only a handful went outside to the parade ground for a roll call and to swear an oath of allegiance to the separatist region.
OSCE:
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.
A Reuters 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, detained days earlier by pro-Russian separatists, had been released in the early hours of Friday.
REUTERS