HORLIVKA, Ukraine -- The fuel is local, but the matches are Russian. That in a nutshell is how the insurgency threatening the survival of Ukraine as a unified state is increasingly unfolding.

Over the past 10 days, more than a dozen government offices in eastern Ukraine have been taken over by pro-Russian forces, with most of the seizures following the same pattern. Aggressive gangs, sometimes carrying firearms and wearing military fatigues, storm the buildings. The Ukrainian flag is replaced with a Russian one. Then local men move in to hold them.

Those capturing the buildings insist they are carrying out the will of the people and have demanded a referendum on autonomy for the eastern Donetsk region. Relatively small numbers have hit the streets in support, however, and it is increasingly evident the purported uprising is far from spontaneous and is being carried out with unerring co-ordination.

Russia has tens of thousands of troops massed along Ukraine's eastern border. Western governments accuse Moscow of fueling the unrest and worry that the spectre of bloodshed could be used as a pretext for a Russian invasion, in a repeat of events in Crimea a few weeks ago.

The Ukrainian government's inability to quash the pro-Russian insurgency was highlighted by acting President Oleksandr Turchynov's call Monday for the deployment of United Nations peacekeeping troops in the east of his country. He said the presence of Russian meddling was clear in the unrest gripping his country.

"The Russian Federation is sending special units to the east of our country, which seize administrative buildings with the use of weapons and are putting the lives of hundreds of thousands of our citizens in danger," Turchynov said, according to the presidential web-site.

Peacekeepers, however, would have to be authorized by the U.N. Security Council, where Russia holds a veto.

Pro-Russian activists point to what they say is an aggressively nationalistic government as justification for their actions. The Cabinet in charge since the overthrow of President Viktor Yanukovych in February includes some nationalist figures, but there is no substantive evidence the Russian-speaking population has been subject to any widespread intimidation in recent weeks.

The relative ease with which pro-Russian groups have been able to overwhelm resistance was in full display Monday in the Donetsk region city of Horlivka.

Hundreds of local people gathered in a square in front of the local police station. Oleksandr Sapunov, who described himself as the head of a public self-defence unit in Horlivka, said the crowd was fighting against appointees of the Kyiv government, including the local police chief.

"The people came to tell him that he is a puppet of the Kyiv junta and they won't accept him," Sapunov said.

Then came the moment for the switchover of the flag from Ukrainian to Russian. But in Horlivka, the head of the police force came to the ledge, chasing the man with Russian tricolour and knocking him 20 feet (6 metres) to the ground, where he banged his head.

The crowd vented its fury. Within seconds, the people hurled stones. The police inside sought to disperse them with stun grenades and by firing guns into the air, but the building was taken within about an hour.

Shortly afterward, men in fatigues marched the chief of police, blood gushing from his head, to an ambulance outside. The onlookers shouted "Donbass! Donbass!" the name of the Donetsk region, and "Ros-si-ya," or Russia.

Details of another assault also followed a similar script.

In the town of Slovyansk, a dozen armed men, many in new-looking military fatigues, swiftly and professionally secured control over the police station Saturday. A day later, local guys wearing regular clothes stood watch, using weapons raided from police stores.

Beyond a barricade blocking the road to the police headquarters, a few hundred people chanted calls for a referendum on autonomy and gave renditions of the Russian anthem.

These episodes have fed into accusations from the government in Kyiv and Western officials that protests are being instigated by the Kremlin and that some of the insurgents are in fact Russian agents.

Boris Grozovsky, a commentator with the independent Russian website Colta.ru, used the fuel and matches analogy to underscore the apparent involvement of outside agents.

"In all the cities in Ukraine where there have been disturbances, it is always outsiders that have acted as the armed attackers and provocateurs," he said.

Russian Foreign Minister Sergey Lavrov again denied Monday that Russian agents were operating in eastern Ukraine, saying it would contradict Moscow's interests. He challenged Ukraine "not to be shy" about backing its claims of capturing Russian security officers with facts.

Eyewitnesses have reported multiple sightings of camouflaged militia similar to those seen in Crimea before that peninsula's annexation and which are widely believed to have been following order from Moscow.

"These armed units ... raised Russian and separatist flags over seized buildings and have called referendums and union with Russia," U.S. Ambassador Samantha Power said during a UN Security Council session Sunday. "We know who is behind this."

Ever since Yanukovych fled to Russia in late February, the Kremlin has demanded constitutional reforms that would turn Ukraine into a loose federal state.

Dmitry Peskov, the spokesman for Russian President Vladimir Putin, was quoted by Russian news agencies as saying Monday that Putin has received "numerous appeals" from eastern Ukraine "asking him to help and interfere in one way or another." Peskov added that Putin was "watching the developments in those regions with great concern" but wouldn't elaborate.

Meanwhile, Putin urged President Barack Obama to discourage the Ukrainian government from using force against protesters in the country's east.

The Kremlin said in a statement following Monday's conversation that the Russian leader rejected the claims of Russian agents' involvement in protests as "speculations based on unreliable information." Putin said the protests vented public anger about the Ukrainian government's reluctance to recognize the interests of Russian speakers in the east.

In a development that could help fuel recurring Russian allegation of U.S. interference, White House press secretary Jay Carney confirmed Monday that CIA chief John Brennan had visited Ukraine over the weekend. The CIA denied Brennan had used his trip to advise Ukrainian authorities on tactical operations.

After refusing demands for a referendum by breakaway supporters in the east, Turchynov indicated Monday that holding a nationwide referendum on the country's status was a possibility and that such a vote could be conducted on May 25, along with presidential elections.

Turchynov expressed confidence that Ukrainians would vote against turning the country into a federation and against its break-up. Russia, meanwhile, has frequently and vocally expressed support for federalization.

A deadline set by the Ukrainian government for pro-Russian gunmen to leave government buildings in eastern Ukraine and surrender weapons passed without incident early Monday, with no immediate sign of any action to liberate any seized buildings.

So far, there's no script for that.

- Danilova reported from Kyiv. Nataliya Vasilyeva in Kyiv and Vladimir Isachenkov in Moscow also contributed.