The gunmen, who turned up at dawn yesterday, took control of the offices in Horlivka, a town of almost 300,000 people, said a Reuters photographer. They refused to be photographed.
The heavily armed men wore the same military uniforms without insignia as other unidentified "green men" who have joined pro-Russian protesters with clubs and chains in seizing control of towns across Ukraine's Donbass coal and steel belt.
Attempts to contain the insurgency by the government in Kiev have proved largely unsuccessful, with security forces repeatedly outmanoeuvred by the separatists.
The West and the new Ukrainian government accuse Russia of being behind the unrest, a charge Moscow denies.
Daniel Baer, the US ambassador to the OSCE, a European security watchdog, told reporters in Vienna: "I think it's very clear that what is happening would not be happening without Russian involvement."
A police official in Donetsk, the provincial capital where separatists have declared a People's Republic of Donetsk, said separatists were also in control of the Horlivka police station, having seized the regional police headquarters earlier in April.
Yesterday's takeover followed the fall of government buildings on Tuesday further east in Luhansk, capital of Ukraine's easternmost province, driving home just how far control over the densely populated region has slipped from the central government in Kiev.
"They've taken them. The government administration and police," the police official said of Horlivka.
The town sits just to the north of Donetsk, where mainly Russian speaking separatists have called a referendum on secession for May 11.