Pro-Kiev protesters on Friday blocked a railway line into Ukraine's rebel-held east to protest against trade with Russian-backed separatists, their spokesman told AFP.
Several dozen pro-Kiev former volunteer fighters as well as members of parliament blocked the main railway into the rebel-controlled part of the Lugansk region, Bogdan Lozytsky said.
"This is a protest with no time limit," Lozytsky said by telephone from the protest site. "We are stopping the flow of smuggled coal and other goods between Ukraine and the occupied territories."
The protesters also plan to block roads in the next few days, Lozytsky said.
Since separatist rebels sparked a conflict with government forces in 2014 in the coal-mining Donbass region bordering Russia, nearly 10,000 have lost their lives on both sides.
An "indefinite" ceasefire declared in late December has failed to entirely stop the violence.
Ukraine in 2015 banned almost all trade with the rebel-controlled areas, prompting a boom in smuggling. The government only made it legal to buy coal from the separatist regions.
The pro-Kiev governor of the Lugansk region, Yuriy Garbuz, warned the action "threatens the energy security of the country," in the height of winter.
Garbuz said the protesters "had blocked empty train cars intended to transport coal into the territory controlled by Ukraine."
"If fuel supplies aren't restarted, heat and power stations in central and Western Ukraine will be left without fuel," he warned.