By Arwa Damon, Lena Kashkarova and Laura Smith-Spark
MARIUPOL, Ukraine (CNN) -- Five pro-Russian activists were killed overnight when Ukrainian forces attacked barricades on the outskirts of Ukraine's southeastern city of Mariupol, a spokeswoman for the pro-Russian camp said Wednesday.
Fifteen other activists were detained by Ukrainian forces, Irina Voropaeva said.
The violence comes amid an escalation of tensions as Ukrainian forces seek to regain control of some of the administrative buildings seized by pro-Russian separatists in a swath of the country's south and east.
The activists briefly abandoned the Mariupol City Council building, according to Voropaeva.
But the security forces stayed only briefly in the building, saying they had been ordered to leave.
The activists reentered and Russian and regional flags went back up, to the cheers of the crowd outside.
Elsewhere in the volatile Donetsk region, an uneasy standoff continued Wednesday between the Ukrainian military and the separatists.
Both sides clashed at the rebel stronghold of Slovyansk on Monday. Ukraine's security services said 30 "heavily armed" militants had been killed in recent days as part of the "anti-terrorist" operation in the area.
As the tensions rise, uncertainty reigns.
The eastern Donetsk and Luhansk regions say they'll hold a referendum on autonomy on Sunday, but there are no visible preparations for a vote.
Meanwhile, the interim government in Kiev plans to hold presidential elections on May 25 -- but acknowledges it has lost control of part of the country.
Russian Foreign Minister Sergey Lavrov on Tuesday said it would be "unusual" to hold a presidential election in the country when the army was being deployed against the population.
In what could be a sign of Moscow preparing to question the legitimacy of the May 25 election if it is unhappy with the process, Lavrov said: "In the situation where they use the army against their own population, it's quite unusual."
"This is not Afghanistan; this is a completely different situation," he said, adding that constitutional reforms promised by Kiev's new leaders would not be implemented in time for the vote.
Lavrov, speaking after a meeting of Council of Europe foreign ministers in Austria, also ruled out holding a second international meeting in a bid to defuse the crisis in Ukraine, saying that the provisions of a first international pact signed in Geneva, Switzerland, last month had yet to be put into force.
The agreement called on all parties to refrain from violence, as well as saying illegal armed groups must disarm and vacate seized public buildings.
Kiev and many in the West believe that the separatists are backed by Moscow and fear that Russian President Vladimir Putin is fomenting trouble to increase his influence in the region.
Russia annexed Ukraine's southeastern Crimea region in March, following a referendum staged while armed pro-Russian groups backed by Russian forces controlled key infrastructure.
But Moscow says right-wing, ultranationalist groups are behind the violence in Ukraine and says it has no direct influence over the pro-Russian groups.
The rising tensions could have an impact far beyond Ukraine's borders, NATO Secretary General Anders Fogh Rasmussen warned on Tuesday.
"Today we are facing the gravest crisis to European security since the end of the Cold War," he told reporters.
"But this is not just about Ukraine. This crisis has serious implications for the security and stability of the Euro-Atlantic area as a whole."
CNN's Arwa Damon reported from Mariupol and journalist Lena Kashkarova from near Donetsk, while Laura Smith-Spark wrote from London. CNN's Marie-Louise Gumuchian, Claudia Rebaza and Kellie Morgan contributed to this report.
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