The fatalities came after a night attack on a protester checkpoint on the outskirts of the city, a Rossiya 24 news channel correspondent reported.
Four cars drove by the checkpoint and opened fire at the local residents holding it, killing three people and seriously injuring another one.
A group of protesters, who had firearms unlike those holding the checkpoint, was called from their camp in the city. They opened fire at the attackers, killing two of them, the report said.
The protesters in the confrontation reportedly captured the attackers’ two cars. Firearms, explosives and aerial photos of Slavyansk were discovered there.
RIA Novosti cites a doctor at the city’s main hospital as saying that four people with gunshots were brought in overnight.
“Apparently, something serious happened,” the doctor said.
The protesters believe that they were attacked by paramilitary from the Right Sector.
Protesters in eastern Ukraine have captured government buildings across the Donetsk region over the past two weeks. They call the authorities in Kiev illegitimate and are demanding a referendum to vote on autonomy for their region. Similar calls are coming from other eastern Ukrainian regions.
Kiev deployed military and special operation troops to the Donetsk region in a bid to crackdown on what they referred to “terrorist actions” by the protesters. The crackdown so far has been futile, with a number of troops switching sides to the protesters.
Russia, Ukraine, the US and the EU signed an agreement this week in Geneva aimed at de-escalating the tension in Ukraine. One of the key points in it is disarming militias and paramilitary units in the country. But neither Right Sector and similar pro-Maidan groups nor the anti-Maidan militias in the east seem to be willing to take the first step.