Pro-Russian separatists reinforced barricades around the state security building in the eastern Ukrainian city of Luhansk on Wednesday and called on President Vladimir Putin for help after the government warned it could use force to restore order.
But protesters were also engaged in talks to ease the standoff, which Kyiv has said could provide a pretext for a Russian invasion, and lawmakers from eastern Ukraine proposed an amnesty for protesters to defuse tension.
The former KGB headquarters is one of three government buildings seized this week in eastern Ukraine by protesters demanding regional referendums on independence from Kyiv, like the one in Crimea that led to its annexation by Russia.
Assistant U.S. Secretary of State Victoria Nuland said on Wednesday that Washington has no doubt that Russians were behind the takeovers of government buildings in eastern Ukraine this week.
"I don't think that we have any doubt the preponderance of direct Russian involvement," Nuland, the top U.S. diplomat for Europe, said at a congressional hearing on the Ukraine crisis.
Tensions have risen in the mainly Russian-speaking east since the overthrow of Ukraine's Moscow-backed president and the installation of a new pro-European government.
Sandbags and wooden crates were piled near the entrance of the building to defend it against the police. Men with rifles could be seen through broken windows above.
"The authorities are trying to discredit us, to discredit and by any means make us turn our rifles against each other," said an unidentified protester said speaking into a microphone from the barricade.
"They are trying to urge our young men, those who served in the army and those who did not serve in the army, they are trying to make them take weapons and go to the border with Russia. We should not allow them to do that,"
Local police spokeswoman Tatyana Pogukai said protesters had found an arsenal of weapons within the building. Protesters say they have 200-300 Kalashnikov automatic rifles. She denied previous reports that hostages had been taken.
She said negotiations had been carried out overnight but the two sides had not come to an agreement.
Ukraine's state security service said that 50 people had left the building in Luhansk overnight. Protesters confirmed that some had left.
Activists, many in balaclavas and masks, continued to build makeshift barricades and prepared petrol bombs.
Ukraine's government says the actions are part of a Russian-led plan to dismember the country, a charge Moscow denies.