Word spread quickly through the few hundred pro-Russian protesters in Donetsk in eastern Ukraine: "The miners are coming!"

The crowd parted as a group of a dozen or so burly men in orange work helmets marched past barbed-wire and tyre barricades into the 11-storey administration building, which protesters seized last weekend as they demanded greater independence from Kiev.

"Glory to the miners!" the crowd began chanting. "Glory to Donbass!" they shouted, much as protesters at Kiev's Euromaidan demonstrations had shouted "Glory to Ukraine!" before they ousted the president, Viktor Yanukovych, in February.

Donetsk is the heart of eastern Ukraine's coalmining country, historically known as the Donbass, and its football club is called the Miners. Cultural and economic ties to Russia – about three-quarters of people in the Donetsk region speak Russian as their native language – have put the Donbass on a collision course with the new government in Kiev, which plans to sign an association agreement with the EU. Yanukovych is from Donetsk and many here still call him the legitimate president.

Collisions were spreading across eastern Ukraine on Saturday night. Armed separatists seized government buildings in Slaviansk and set up barricades on the outskirts of the city in what Kiev described as an "act of aggression by Russia". The developments have increased concerns of a possible "gas war" that could disrupt energy supplies across the continent.

Militants also took control of the police headquarters in Kramatorsk, 95 miles from the Russian border, after a firefight. Video footage showed an organised unit of more than 20 men wearing matching military fatigues and taking orders from a commander while shooting automatic rifles as they approached the building.

The White House said it will send Vice President Joe Biden to Kiev on April 22 to demonstrate high-level US support for Ukraine after expressing concern about escalating tensions in the eastern part of the country.

Back at the Donetsk occupation, the hundreds of supporters who have gathered each day are a small number of the city's nearly one million residents. But if the 100,000-plus employees of coalmining enterprises were to rise en masse, that would change the political picture drastically, in a similar fashion to the Donbass miners' strikes that helped bring about the breakup of the Soviet Union.

"It's hard to arouse the miners, but when you do, there will be trouble," said Artyom, a former miner who was guarding the administration building on Friday night. "If the miners all rise up, it will be an economic, physical and moral blow. It will be hard for everyone."

Protesters have declared the administration building a "people's republic". In the neighbouring coalmining region of Luhansk, the "army of the south-east", a group of armed men, including former Berkut riot police who fought protesters in Kiev, has occupied the security service headquarters and demanded a referendum. The protesters want economic and political independence from Kiev and many support a federalisation of the country. But they have also called on Russia to send in peacekeeping forces.

Although many of the protesters occupying the Donetsk administration building wear masks, the miners among them are easy to spot. The edges of their eyelids are black from coal dust, as if rimmed with kohl, giving them an exotic look. For now the contingent is limited: the group greeted by the crowd last Thursday included a reported 200 miners, who said they had come to defend the building from a rumoured assault by the authorities. But most miners support greater independence from Kiev, according to those present.

"There's only one position, only in support of the referendum," said a miner who identified himself only as Vitaly. "But we can't stop working today, or tomorrow I'll be on the street," he added, saying that any strike would put the mine out of commission for a significant period.

Oleg Krymenko, another local miner, said he did not support the occupation but worried about rising prices – the cost of utilities and basic goods has been shooting up in recent months – and said ties with Russia should be close. "They work and that's it. Before their shift, they have to relax. Coalminers don't engage in nonsense," he said about the protests.

A miner's work is tough, especially in the ageing coal mines of the Donbass. Local miners descend to depths of up to 1,300 metres and often work in temperatures pushing 100 degrees fahrenheit. Fatalities are common, and 111 died in a series of explosions at the local Zasyadko mine in 2007. Flags were lowered to half-mast in Donetsk on Friday after seven miners died in a gas explosion at the Skochinsky mine.

Equipment is often worn-out and safety procedures are frequently violated, according to Oleg Obolents, a retired miner who recently formed an independent miners' union to fight for better pay and safety standards. Donbass miners are "breathing incense", he said, using an expression that refers to the incense burned during Russian Orthodox funeral services and is roughly equivalent to having "one foot in the grave".

A local miner named Andrei said he came to the barricades every day after work, wearing his orange helmet and headlamp. He and his comrades often discussed the political situation when descending into their mine outside the city, he said.

"We need to fight for our rights and protect the Donbass from Bandera supporters. I don't like the Kiev regime," he said, referring to Stepan Bandera, a second world war nationalist leader who is commemorated with dozens of monuments in western Ukraine but widely reviled as a Nazi collaborator in the east. Many protesters see the new Kiev government as dominated by nationalists from western Ukraine, which has a largely agrarian economy.

But Donbass coalminers were also spotted on the barricades in Kiev before Yanukovych's ousting, showing that not all of them are anti-Kiev.

Andrei's mother Nelya, who also volunteers at the protest, said she wants Donetsk to remain part of Ukraine but with greater local control and resumed trade with Russia.

"This is my child. If he's without work, he won't start a family, and I will be hanging on his neck when I retire. His grandmother, too," she said.

A tenth of Ukraine's coal production is sold to Russia, the country's largest trading partner. Another third goes to power metalworking plants, which also sell much of their product to Russia. But as Ukraine has been gripped by political crisis and the new Kiev regime has turned towards Europe, Russia has disrupted trade at the border, and orders from Russian companies have fallen off, miners reported.

Although Andrei has continued to receive his monthly salary of 4,000 hryvnia (£190), the Ilyicha metalworking factory in nearby Mariupol, which buys most of his mine's coal, recently ceased operations because orders from Russia dropped off, he said. Before the protests in Kiev, he was making 5,000 hryvnia a month, he said.

If the association agreement goes through and Ukraine's market is completely opened up to Europe, the tougher EU production standards will result in the decline of Ukraine's coal and metallurgy industries and the loss of jobs, Obolents said.

He said that many in the industry had not come out to support the protests, for fear of losing their jobs.

"When they haven't received their pay for two or three months, they'll come out, or if there is a storming [of the occupied building]," he added. "The hungry have nothing to lose. The hungry are willing to do anything."

Most of the major mines in Donbass are owned by Rinat Akhmetov, Ukraine's richest man, who has served as a mediator in negotiations between the Donetsk protesters and the Kiev-appointed governor. In a speech on Friday at a meeting with Ukraine's prime minister, Arseniy Yatsenyuk, Akhmetov said he supported the protesters' demands for preserving the Russian language and greater independence from Kiev, but added: "For me, Donbass is Ukraine."

Valera, a miner who said the Kiev regime was cracking down on the Russian language, predicted "trouble for the bosses" if the mines stopped working. "If they stop, there will be war," he said.