* Gunmen in military fatigues without insignia
* Control government building in eastern Horlivka
* Ukraine's acting president says police helpless (Adds details, colour; changes dateline, previous KIEV)
By Marko Djurica
HORLIVKA, Ukraine, April 30 (Reuters) - Masked gunmen in military fatigues took control of a government building in another Ukrainian town on Wednesday, as pro-Russian separatists tightened their grip on a swathe of the country's industrial east largely unopposed by police.
Local media reports said the gunmen turned up at first light, and were later seen by a Reuters photographer to be controlling entry to the building in the town of almost 300,000 people. They refused to be photographed.
The heavily armed men wore the same military uniforms without insignia as other so-called "green men" who have joined pro-Russian protesters with clubs and chains in seizing control of a string of towns across Ukraine's Donbass coal and steel belt abutting the border with Russia.
A police official in nearby Donetsk, the provincial capital where separatists have declared a "People's Republic of Donetsk", said separatists were also in control of the Horlivka police division, having seized the regional police HQ earlier in April.
Wednesday'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 pro-Western central government in Kiev.
"They've taken them. The government administration and police," the police official said of Horlivka.
The town sits just north of Donetsk, where mainly Russian-speaking separatists have called a referendum on secession for May 11.
Many hope to follow Crimea's break from Ukraine in late March and subsequent annexation by Russia, following the overthrow of Ukraine's then Moscow-backed president Viktor Yanukovich in late February in a tug-of-war between the West and Russia over the strategic direction of the former Soviet republic.
The Donbass region is home to giant steel smelters and heavy plants that produce around a third of Ukraine's industrial output.
An armed uprising began there in early April, with Kiev almost powerless to respond for fear of provoking an invasion by tens of thousands of Russian troops massed on the border.
Oleksander Turchynov, Ukraine's acting president until an election on May 25, reiterated on Wednesday that police were incapable of reasserting control in the region.
"Our main task is to prevent the terrorist threat from spreading to other regions of Ukraine," he told a meeting of regional governors in Kiev.
"The Russian leadership is doing everything to prevent the election. But the election will take place on May 25," he said.
Authorities in Kiev said security forces had "liquidated" three separatist checkpoints near the eastern town of Slaviansk, a separatist stronghold, and the gunmen manning them had disappeared. The information could not be independently confirmed. (Additional reporting by Pavel Polityuk in Kiev; Writing by Matt Robinson; Editing by Giles Elgood)