Protesting Ukrainian war veterans and opposition lawmakers have blocked rail freight traffic between government and rebel territory since January to stop the insurgents from receiving the cash they get in trading with Kiev.
The situation has riled Ukraine's pro-Western government as it cut off access to a specific type of coal produced only in the east that it requires.
But the activists accuse the authorities in Kiev of allowing the trade to indirectly fund rebels in a 34-month war that has claimed more than 10,000 lives and continues to grind on.
The first person to suffer from the threat was Ukraine's richest man. Rinat Akhmetov is a billionaire whose empire include the telephone provider Ukrtelecom.
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Akhmetov's holding company SCM said today that Ukrtelecom's office in the industrial eastern regions controlled by the militias had been seized.
"As of Wednesday, Ukrtelecom announces the forced interruption of services in the part of the Donetsk region not under Kiev's control," it said in a statement.
He added that his humanitarian assistance group in the region was also affected.
"Due to armed people's blockade of all centres of The Rinat Akhmetov humanitarian fund... Hundreds of thousands of the elderly, the handicapped and young mothers have been left without humanitarian aid."
Insurgent leader Alexander Zakharchenko said that the plan to take over enterprises was "going according to plan".
"Since midnight we are introducing (our own) control," he was quoted as saying by the rebels' news agency.
Neighbouring rebels in the self-proclaimed Lugansk People's Republic said they had seized three Akhmetov-owned coal mines this week.
Their operations continued but the mines' takeover may create serious problems for the tycoon and coal-dependent Ukraine.
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