Seven international military observers taken captive in eastern Ukraine a week ago have been released.
Five Ukrainian officers captured with the observers, who are linked to the Organization for Security and Co-operation in Europe, were also freed.
Pro-Russian separatists in the town of Sloviansk say they released the OSCE observers "without conditions".
Kiev has resumed military action against the separatists, with fighting reported in some areas.
In Kramatorsk, south of rebel-held Sloviansk, Ukrainian forces have recaptured the headquarters of the SBU security service from pro-Russian separatists, according to the interior ministry.
It said the building was now held by the national guard.
Interior Minister Arsen Avakov earlier said the "active phase" of the military operation against pro-Russian rebels continued at dawn, with Ukrainian forces taking a television tower in Kramatorsk.
"We are not stopping," Mr Avakov wrote on his Facebook page.
BBC correspondent Sarah Rainsford says she spoke to residents of the area on Friday night and was told some in Kramatorsk had gone out to try to stop armoured personnel carriers from entering.
The defence ministry also reported "serious fighting" on Friday night in Andreevka, some 6km (4 miles) west of Sloviansk, with two Ukrainian soldiers killed.
Ten separatists were killed in Andreevka, a spokeswoman for the pro-Russian force in Sloviansk said.
The military operations come a day after running battles between pro-Kiev groups and separatists in Odessa, which culminated in a deadly fire at the Trade Unions House.
At least 42 people were killed, most of them in the blaze.
Both the OSCE and Vyacheslav Ponomaryov, a leader of the insurgency in the east, confirmed the release of the international monitors on Saturday.
"As I promised them, we celebrated my birthday yesterday and they left. As I said, they were my guests," Mr Ponomaryov said.
Russia had sent an envoy to negotiate the releases. Vladimir Lukin said he hoped the "voluntary act" by the separatists would be reciprocated by Kiev, adding: "I would very much like military actions to end."
President Vladimir Putin's spokesman in Moscow, Dmitry Peskov, said Moscow had no sway over the separatists: "From now on Russia essentially has lost its influence over these people because it will be impossible to convince them to lay down arms when there's a direct threat to their lives."
One of the observers, German Col Axel Schneider, said the team had been treated "as good as possible" in what was a "miserable situation".
Western leaders had condemned the abductions.
The observers - four Germans, a Dane, a Pole and a Czech - are not part of the main OSCE monitoring mission, which was agreed after long negotiations by Russia, Ukraine and the US.
Meanwhile, in southern Ukraine, flowers were placed outside the burned out trades union building in Odessa where at least 36 people had lost their lives the day before.
Reports suggest pro-Russian separatists had barricaded themselves inside the building and both sides were throwing petrol bombs.
The deaths sparked another war of words involving the West, Russia and Ukraine.
Ukraine's secret service said groups from Russia and Moldova's breakaway region of Trans-Dniester were fomenting the unrest.
But Mr Peskov said: "Kiev and its Western sponsors are practically provoking the bloodshed and bear direct responsibility for it."
Local police chief Petro Lutsiuk told Reuters news agency that more than 130 people had been arrested following the violence in Odessa.W