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A Reuters photographer said a Ukrainian Air Force helicopter had fired on the outskirts of the city, which has a population of more than 100,000 people.
A reporter also said he heard gunfire in the city.
An aide to Ukrainian Interior Minister Arsen Avakov said he could not comment "until it's over."
Slovyansk has been firmly under the control of the separatist forces since last month when activists first took control of the police headquarters.
They have since taken the city hall and other government buildings, ITAR-TASS reports.
The pro-Russian rebels in the city have detained Slovyansk's mayor, several journalists, three Ukraiian Security Service (SBU) members, and are also holding Organization for Security and Cooperation in Europe (OSCE) military observers, including four Germans.