Pro-Russian activists in Ukraine's industrial centre of Donetsk have proclaimed their independence from Kiev and pledged to hold a referendum in the next month, provoking fears that Moscow could be orchestrating a second Crimea scenario in Ukraine's east.

"Seeking to create a popular, legitimate, sovereign state, I proclaim the creation of the sovereign state of the people's republic of Donetsk," said a man into a loudspeaker outside the seized regional administration building to a cheering crowd.

The protesters said they would hold a referendum no later than 11 May on the region's status, and also asked Russia to ready "peacekeeping troops", in a scenario reminiscent of the events that led to the annexation of Crimea last month.

In Kiev, the interim prime minister, Arseniy Yatsenyuk, said events in the east were being carried out according to a script written in Moscow.

"An anti-Ukrainian plan is being put into operation … under which foreign troops will cross the border and seize the territory of the country," Yatsenyuk told a cabinet meeting in Kiev. "We will not allow this."

Russia is believed to have massed tens of thousands of troops on the border. The German chancellor, Angela Merkel, said last week that Putin told her those troops would be removed, but on Monday, Daniel Baer, US ambassador to the OSCE, said there was no sign of this.

"We have strong evidence that there are tens of thousands of forces on the border and again not in their normal peacetime positions or garrisons," he said.

The White House claimed pro-Russian demonstrators in eastern Ukraine were paid outsiders, but declined to specify who it believed provided the money, simply blaming Moscow in general for renewed "provocation"

The White House spokesman, Jay Carney, told reporters: "We are concerned about several escalatory moves in Ukraine over the weekend. We see these as a result of increased Russian pressure on Ukraine. We saw groups of pro-Russian demonstrators take over government buildings in the eastern cities of Kharkiv, Donetsk and Luhansk.

"If Russia moves into eastern Ukraine, either overtly or covertly, this would be a very serious escalation. We call on President Putin and his government to cease all efforts to destabilise Ukraine and we caution against further military intervention."

Writing in the Guardian, however, Russia's foreign minister, Sergei Lavrov, says it is the west, and not Russia, that is guilty of destabilising Ukraine. He says the EU and US have been pursuing an "unproductive and dangerous" policy in the region.

"They have been trying to compel Ukraine to make a painful choice between east and west, thus further aggravating internal differences," writes Lavrov, adding that Russia wants Ukraine to carry out constitutional reform and guarantee its status as a non-aligned state that will never accede to Nato.

"We are not imposing anything on anyone. We just see that if it is not done, Ukraine will continue to spiral into crisis with unpredictable consequences," writes Lavrov.

Russia's foreign ministry said it was "carefully observing" events in the east and south of Ukraine, and again called for "real constitutional reform" to turn the country into a federation. Moscow told Ukraine to stop blaming Russia for its problems.

At a meeting with security chiefs in Moscow on Monday, Putin called for vigilance against foreign-funded NGOs promoting political unrest in Russia.

"We will not accept a situation like what happened in Ukraine, when in many cases it was through non-governmental organisations that the nationalist and neo-Nazi groups and militants, who became the shock troops in the anti-constitutional coup d'etat, received funding from abroad," he said.

How much current events in eastern Ukraine are under the control of the Kremlin is unclear, but when Putin appealed to the Russian parliament for authorisation to use troops in Ukraine in February, the resolution did not specify that it applied to Crimea only, leaving the door open for a further incursion into the east.

In Donetsk and other eastern Ukrainian cities, which are largely Russian-speaking, there is real discontent with the new government in Kiev, which has been in power since President Viktor Yanukovych fled at the culmination of months of street protests. Nevertheless, the region is far less pro-Russian than Crimea, and analysts say Russia would find it harder and more complicated to introduce troops there.

In Donetsk on Monday, the regional administration centre was surrounded with tyres and razor wire and the Russian flag was flying. In Lugansk, another eastern city, activists seized a building beloning to security services. In Kharkov, two protests, one in favour of Ukrainian unity and the other pro-Russian, clashed across police lines in the afternoon.

The protests are believed to be being carried out by a small minority in each city. Nevertheless, Russian complaints that the east and south of the country are not being given a voice in the new government have been echoed by some in Kiev itself.

Mustafa Nayem, a journalist who was one of the instigators of the protest that led to the ousting of Yanukovych, complained that the new government had not done enough to reassure the east. "Since Viktor Yanukovych fled, more than a month has passed, and during that time not a single leader from the new government has been to Donetsk, Kharkov or Lugansk. It is unclear who is representing the interests of people in these regions at a national level," he wrote on Facebook on Sunday. He said the government should hold cabinet meetings in the east, visit local elites and involve them in decision making, rather than "sitting in Kiev like cowards".

Also on Monday, Yulia Tymoshenko, the former prime minister who is standing in presidential elections planned for 25 May, flew to Donetsk for consultations.

Ukraine is already reeling from losing the Crimea peninsula to Russia, where there were reports in the morning that a Ukrainian army officer had been shot dead by a Russian soldier. Russia now has full control over the region, and Ukraine's natural resources minister estimates that Ukraine lost $10.8bn of natural resources and other assets in the annexation.

Russia, meanwhile, has claimed that Ukraine has accrued a de facto debt of $11bn after the cancelling of a gas discount that it says was given in advance. Even Ukraine admits that it has a $2.2bn debt for gas, and Gazprom said it had not received any money towards this amount of, nor any payment for March deliveries. It is unclear what the next step will be and whether Gazprom will turn off supplies.

Last week Gazprom raised the price by 80% to $485 per 1000 cubic metres, meaning Ukraine would pay a higher price for gas than western European countries. Yatsenyuk called the price "political" and said Ukraine would not pay.