Armed pro-Russian militants raised the Russian flag in an eastern Ukrainian city on Saturday, deepening a stand-off with Moscow which, Kiev warned, was dragging Europe closer to a "gas war" that could disrupt supplies across the continent.
At least 20 men armed with pistols and rifles took over the police and security services headquarters in Slaviansk, about 150 km (90 miles) from the border with Russia.
Officials said the men had seized hundreds of pistols from arsenals in the buildings. The militants replaced the Ukrainian flag on one of the buildings with the red, white and blue Russian flag.
Some local residents helped the militants build barricades out of tires in anticipation that police would try to force them out, a Reuters photographer at the scene said. But there was no sign that any police action was imminent.
The occupation is a potential flashpoint because if the militants are killed or hurt by Ukrainian forces, that could prompt the Kremlin to intervene to protect the local Russian-speaking population, a repeat of the scenario in the Crimea region when Russian troops were sent in.
Russia denies any plan to send in forces or split Ukraine, but the Western-leaning authorities in Kiev believe Russia is trying to create a pretext to interfere again. NATO says Russia's armed forces are massing on Ukraine's eastern border, while Moscow says they are on normal maneuvers.
Ukraine's acting foreign minister Andrii Deshchytsia said he had spoken in a phone call with Russian Foreign Minister Sergei Lavrov and demanded Moscow stop what he called "provocative actions" by its agents in eastern Ukraine.
Russia and Ukraine have been locked in confrontation since protests in Kiev forced the Moscow-backed president from office, and the Kremlin sent troops into Crimea.