Ukrainian forces have retaken the security service headquarters in the eastern town of Kramatorsk, south of rebel stronghold Slaviansk, the country's Interior Ministry said on Saturday.
"Now it is under the control of the National Guard," the ministry said in a statement. Local television showed pictures of armored personnel carriers moving through the town.
This comes after the Organization for Security and Cooperation in Europe (OSCE) confirmed that a group of military observers, who were seized last week by pro-Russian separatists in the eastern Ukrainian city of Slaviansk, has been released.
"We can now confirm the release of the military observers," said a spokeswoman for the OSCE, under whose auspices the military verification officers led by German defense ministry staff were in Ukraine, at Ukraine's request.
"Staff of the special monitoring mission are now waiting to receive them," she said, referring to a larger OSCE mission gathering information and trying to reduce tensions on the ground in Ukraine.
Earlier, Slaviansk's pro-Russian separatist leader said he had released the observers, and said the move was voluntary and made to coincide with his birthday.
Meanwhile, Ukraine said military operations against pro-Russian separatists in the industrial east of the country continued at dawn on Saturday near the town of Kramatorsk.
Interior Minister Arsen Avakov said Ukrainian forces had seized control of a television tower in Kramatorsk, near the rebel stronghold of Slaviansk where there was heavy fighting on Friday. "We are not stopping," Avakov wrote on his Facebook page, but gave no information on casualties.
This as Ukraine's security service said illegal military groups from Moldova's breakaway region of Transdniestria and Russian groups worked together to foment unrest in the southern port city of Odessa.
"The unrest, which occurred on May 2 in Odessa and led to clashes and many casualties, was due to foreign interference," a spokeswoman for the SBU security service told a news conference.
She said former top officials, once part of ousted Ukrainian President Viktor Yanukovich's inner circle, had financed "saboteurs" to foment the unrest, blaming Serhiy Arbuzov and Oleksander Klymenko, who were now "hiding in a neighboring country".
On another note, Russian President Vladimir Putin's spokesman said that both the Kiev authorities and their backers in the West were directly responsible for bloodshed in the Ukrainian city of Odessa, Russian new agencies reported.
"Kiev and its Western sponsors are practically provoking the bloodshed and bear direct responsibility for it," RIA Novosti quoted spokesman Dmitry Peskov as telling reporters.
Peskov also said Friday's violence, in which dozens died, made the idea of holding presidential elections in Ukraine later this month "absurd".
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Russia's Foreign Minister Sergei Lavrov has told his U.S. counterpart John Kerry that the U.S. should use its influence to make Ukraine's government immediately stop military operations in south-east Ukraine, the Russian foreign ministry said in a statement on Saturday.
Lavrov also said that it was important that the mediating role of the OSCE was increased to secure Kiev's fulfillment of the Geneva declaration on de-escalating tensions in Ukraine.
"Chances of this still exist," the statement said, as long as all Ukrainian regions are represented in a national dialogue on constitutional reform, and "terrorists" from the Right Sector group are curbed.
This as, Russia's Lavrov and the Swiss president Didier Burkhalter "confirmed the need for complete fulfillment of Geneva declaration" on Ukraine, the Russian foreign ministry said in a statement.
Burkhalter is also presently the chairman of the Organisation for Security and Cooperation in Europe.
The statement followed a phone conversation between Lavrov and Burkhalter on Saturday where they discussed the situation in Ukraine and especially "the rampage of radicals" in Odessa, the statement said.
Russia's Foreign Ministry also said in a statement that the release of OSCE observers from European countries shows the "bravery and humanism" of the defenders of Slaviansk.
For his part, U.S. Secretary of State John Kerry welcomed the release in Ukraine of military monitors from the Organization for Security and Cooperation in Europe (OSCE) but said more steps were needed to de-escalate the crisis.
REUTERS