Pro-Russian separatists in eastern Ukraine rejected an offer of amnesty from the government in Kiev on Tuesday, raising fears of an armed confrontation that could, in turn, prompt Russia to intervene.

Earlier this week, activists in Luhansk, a city close to the Russian border, occupied a former secret police headquarters there. Another government building was seized in Donetsk, an industrial city further south. In both places, activists said they would lay down their weapons only if the government agreed to call a referendum on the regions’ future status.

The demands echoed those of Crimean separatists who declared independence from Ukraine last month before joining Russia.

The authorities in Kiev, who refused to recognize the Crimean referendum for session, had offered the occupants amnesty but ruled out plebiscites in eastern Ukraine, saying the occupations were part of a Russian plot to dismember the former Soviet republic.

“We are trying to find a compromise but the demands put forward by the occupiers are unacceptable,” Deputy Interior Minister Serhiy Yarovy told reporters. “Our aim is to avoid the use of force but that option remains in place.”

Some of the activists in Luhansk told Britain’s The Guardian newspaper they would return fire if security forces tried to remove them from the buildings. They identified themselves as former special police officers from other parts of the country.

After seizing the Crimea, Russia suggested Ukraine should be turned into a federal state. Sergei Lavrov, President Vladimir Putin’s foreign minister, said last month, “We are convinced that deep constitutional reform is required. Frankly speaking, we do not see any other way for sustainable development of the Ukrainian state other than a federal state.”

He also said the ouster of President Viktor Yanukovich in February — which came after months of protests against his decision to pull out of an association agreement with the European Union and deepen ties with Russia instead and ignited pro-Russian sentiment in the east — was “the result of a deep crisis of national identity” caused by Ukraine’s inability to reconcile the interests of its various regions.

Before the Crimean annexation, the Russian Senate had given Putin permission to use force in the whole of Ukraine in order to protect the lives of Russian citizens and their “compatriots” there.

Secretary General Anders Fogh Rasmussen of NATO, visiting Prague, accused Russia of “stirring up ethnic tensions in eastern Ukraine and provoking unrest.” He added, “Russia is using its military might to dictate that Ukraine should become a federal, neutral state. That is a decision which only Ukraine as a sovereign state can make.”

Ukraine’s acting president, Oleksandr Turchynov, similarly said in a television address on Monday, “This is the second wave of the Russian Federation’s special operation against Ukraine. Its goal is to destabilize the state and overthrow Ukrainian power, disrupt the elections and tear our country into pieces.”

Two polls published last month put support for federalization at 14 to 15 percent among Ukrainians — and that included the Crimeans who are now Russian citizens. Support was higher in the east and south, where most of the country’s Russian speakers live. Even there, though, only a quarter of residents were in favor of such a decentralization scheme.

NATO released satellite images on Thursday of what it said was a Russian military buildup on Ukraine’s eastern frontier. The pictures showed rows of hundreds of armored vehicles and tanks apparently waiting for orders fifty kilometers from Ukraine’s borders as well as fighter planes and helicopters which the alliance said could be ready to move within twelve hours.

Russia claimed the images were from an exercise last year.