The US issued a stark warning on Tuesday that Moscow could be orchestrating another Crimea-style intervention in eastern Ukraine, after pro-Russian forces cemented their control of a government building in the city of Donetsk and there were claims that in Lugansk protesters had taken up to 60 people hostage.

The US secretary of state, John Kerry, described recent developments in eastern Ukraine as "more than deeply disturbing" and repeated threats of western sanctions against Russia, which he said would be extended to the country's energy, banking and mining sectors.

Kerry blamed pro-Russian protests in eastern Ukraine on "special forces and agents" sent by Moscow in a move he said "could potentially be a contrived pretext for military intervention just as we saw in Crimea".

He was speaking to the Senate foreign relations committee as Ukraine's state security service said pro-Russian separatists had placed explosives in a building they seized in the eastern city of Lugansk on Sunday and were holding about 60 people against their will.

The protesters, who are believed to have weapons, denied they had taken any hostages.

Meanwhile, protesters in Donetsk, who have taken over a regional building and are calling for independence from Kiev, fortified the building with barricades of barbed wire and tyres. In Kharkiv, Ukrainian police detained 70 pro-Russian demonstrators who had occupied a regional government building. Hundreds of angry pro-Russian protesters rallied in front of the building on Tuesday afternoon, demanding the release of those arrested.

"When there was the protest in Kiev everyone called it a peaceful protest, so why aren't we allowed to do the same thing here?" said Olga Maleyeva, a 52-year-old businesswoman in the Kharkiv crowd. People threw eggs and stones at a police bus as it drove past.

Kerry accused Russia of fomenting unrest in the east."Everything that we've seen in the past 48 hours from Russian provocateurs and agents operating in eastern Ukraine tells us that they've been sent there determined to create chaos," he said. "That is absolutely unacceptable. These efforts are as ham-handed as they are transparent, frankly."

The Ukrainian foreign ministry also said the separatist movements in Kharkiv and Donetsk were led by Russian security services. It described events as setting the scene for "the second stage of the occupation of Ukraine", after the annexation of Crimea by Moscow last month.

In return a Russian foreign ministry statement claimed that about 150 operatives of Greystone, a division of the controversial US private security firm Blackwater, were operating in Ukraine disguised as local law enforcement officers, an allegation the US ambassador in Kiev dismissed as "rubbish".

Kerry said: "Quite simply, what we see from Russia is an illegal and illegitimate effort to destabilise a sovereign state and create a contrived crisis with paid operatives, across an international boundary, engaged in this initiative." He added that while the west's preference was for a diplomatic de-escalation of the crisis of the Ukraine, Washington was ready "to do what is necessary" to maintain international order.

He gave no specifics, but repeated previous threats of expanded sanctions, some of which are already targeting officials close to the Russian president, Vladimir Putin, over the attempted annexation of Crimea. Referring to aspects of Russia's economy that could be targeted he added: "Energy, banking, mining – they're all on the table."

The Nato secretary-general, Anders Fogh Rasmussen, also urged Russia to be cautious. "If Russia were to intervene further in Ukraine it would be a historic mistake," he told a news conference in Paris. "It would have grave consequences for our relationship with Russia and would further isolate Russia internationally."

Events have mirrored the initial situation in Crimea, which began with the seizure of the local parliament and ended with Russian annexation. There have been several waves of unrest in the east, though support for integration with Russia is nowhere near as strong as it was in Crimea. Despite the tens of thousands of troops allegedly poised on the border, it is not clear whether Russia is planning an incursion or is merely trying to put pressure on Kiev to accede to its demands for the federalisation of Ukraine.

Putin plans to meet the Russian cabinet on Wednesday to discuss economic ties with Ukraine. Russian energy group Gazprom says Kiev owes it at least $2.2bn (£1.3bn) and has not paid anything for gas deliveries in March. Gazprom raised the price for gas to Ukraine by 80% last week, a move described in Kiev as political. Putin's spokesman did not say whether the cabinet would discuss discontinuing gas supplies to Ukraine.

Russia said it was prepared to take part in four-way talks with Washington, Kiev and the EU, but said representatives of south and east Ukrainian regions should be present at any such talks.

On Tuesday night the Russian ministry said in a statement that the Russian foreign minister, Sergei Lavrov, and Catherine Ashton, the European Union foreign policy chief, had discussed possible international efforts to find a solution to the crisis, and that Lavrov reaffirmed Russia's proposal for "an authentic Ukraine-wide dialogue involving all political forces and regions" aimed at reaching agreement on constitutional reforms. The US, the EU, Russia and Ukraine will meet next week to discuss the crisis, with Kerry, Ashton, Lavrov and Ukraine's Foreign Minister Andriy Deshchytsia all in attendance, an EU official said..