Highlighting that risk, Mr. Kolomoisky in his remarks to reporters noted that on his command, 2,000 armed men could be brought to Kiev within hours. Still, the commander of Mr. Kolomoisky's main paramilitary group, Dnepro-1, denied any involvement.
By Monday, no armed men were visible outside, though the group loyal to Mr. Kolomoisky apparently still occupied the building. Valentin Nalivaichenko, the director of the security service, told reporters on Monday that his agency would help the police arrest the men occupying the building.
"We confirm that the police and journalists have noticed illegal actions by people with weapons" in the capital, Mr. Nalivaichenko said. "We have a strict order from the president that every person in UkrNafta be disarmed."
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In another sign of mounting tensions, Mr. Nalivaichenko said his agency had also questioned two subordinates of Mr. Kolomoisky in the Dnipropetrovsk governor's office, about their possible roles in the murder of one Ukrainian security agent and in the kidnapping of another.
Dnipropetrovsk is widely considered Ukraine's most important industrial region, and its capital of the same name, located about 300 miles southeast of Kiev, is the country's fourth-largest city. Mr. Kolomoisky was one of several oligarchs, considered too rich to bribe, who were appointed to leadership positions in a bid to stabilize Ukraine.
In a statement posted on his website Monday in response to the standoff, Mr. Poroshenko said the volunteer battalions should be "vertically integrated" into Ukraine's regular army, which the government has been struggling to rebuild.
Mr. Kolomoisky, widely known as a pugilistic character even as he is admired for his patriotism, has shown no signs of backing down. In an interview on the 1+1 television station, which he owns, Mr. Kolomoisky said he had spoken to Mr. Poroshenko and they had agreed "that this is not the way this should happen."
Critics of Mr. Kolomoisky, however, said his actions showed his first allegiance was to his own wealth. Mustafa Nayem, a young member of Parliament from Mr. Poroshenko's party, urged the president and Prime Minister Arseniy P. Yatsenyuk to oust Mr. Kolomoisky.
"Igor Kolomoisky has no right to wear the title of a public servant," Mr. Nayem wrote in a blog post. "And the president and prime minister have all the levers to correct the error."