Russia cut off gas to Ukraine today in a dispute over unpaid bills that could disrupt supplies to the rest of Europe and set back hopes for peace between the former Soviet neighbours.
After the weekend loss of 49 troops when pro-Russian rebels shot down a military transport plane, Ukraine's new president ordered his forces to retake full control of the border with Russia.
He said this could then pave the way for negotiations.
Calling time on weeks of wrangling in talks over natural gas supplies, Russia said Ukraine had missed a deadline this morning to repay $1.95bn owed for previous purchases and announced Ukraine would now only get gas it has paid for in advance.
At the same time, Russia insisted that Ukraine must let Russian gas flow across the country through international pipelines to Russia's clients in the European Union.
It noted there could be a temptation for Ukraine to tap into those supplies in transit.
Ukraine and Russia blamed each other for the failure to agree on the price of future gas deliveries and refused to abandon well established positions: Russia offering a discount and Ukraine rejecting that as a tool for political manipulation.
The talks are bound up with the worst crisis between Russiaand Ukraine since the Soviet Union collapsed - a crisis that has brought Western sanctions on Moscow, the Russian annexation of Crimea and Cold War-style confrontations along the borders.
Western-backed Ukrainian President Petro Poroshenko, elected last month to replace the Kremlin-friendly leader ousted in February, said today he wanted troops to regain full control of the border with Russia this week.
After that, there could be a ceasefire and efforts to come up with a peace plan.
"The ceasefire will be declared as soon as the border is secure," Mr Poroshenko told his security chiefs.
"Declaring a ceasefire while the border is open would be irresponsible."
His remarks underlined his concern that Russia is supporting the rebels by sending in tanks, guns and men.
Hopes of a lowering of tension had already been dented before the gas talks failed by the downing of the plane near the eastern frontier, an attack on Russia's embassy in Kiev and new accusations from NATO that Russia is arming the Ukrainian rebels.