Ukrainian war veterans and their supporters say they will step up a trade blockade of rebel-held regions of industrial eastern Ukraine.
For a month about 100 activists have blocked railway lines to stop Ukraine trading with pro-Russian rebels.
The Donetsk region is a key supplier of coal to Ukrainian power stations and heavy industry.
The "blockade headquarters" made the new threat on Facebook, and halted talks with Ukraine's prime minister.
The Ukrainian government has warned that the blockade could lead to power cuts in Ukraine. Steel plants in the east have already been forced to suspend production.
The blockade was imposed to stop Ukraine's so-called "trade in blood" with the rebel-held areas.
More than 10,000 people have died since the separatist rebels seized a large swathe of Donetsk and the Luhansk region in April 2014, triggering clashes with Ukrainian forces. Sporadic fighting continues despite a 2015 ceasefire deal signed in Minsk.
In March 2014, Russia annexed Ukraine's southern Crimea peninsula.
The "blockade headquarters" announced plans "to immediately expand the area of the blockade, cutting off the last routes where the trade in blood is still going on".
They also say the rebels must release Ukrainian prisoners they are holding. A number of Ukrainian MPs are backing the blockade.
On Monday, the rebels threatened to take control of Ukrainian-run businesses if the blockade was not lifted by Wednesday.
"These enterprises will be oriented towards the Russian Federation, to receive raw materials [from Russia]," said Alexander Zakharchenko, leader of the self-declared Donetsk People's Republic (DNR).
Ukraine is dependent on anthracite from the rebel-held area, while Donetsk and Luhansk need coke produced in Ukraine's government-controlled regions.
Last week the mining and metallurgical giant Metinvest suspended production at its main enterprises in the rebel-held east. The company is owned by Ukraine's richest businessman, Rinat Akhmetov.
Russian presidential spokesman Dmitry Peskov said the blockade was "destabilising the situation and hampering the process of settlement and de-escalation".
Ukraine and Western governments accuse Russia of sending reinforcements and heavy weapons to the rebels - something Moscow denies.