Russia cut off gas supplies to Ukraine on Monday, a move that will inflict pain on its turbulent neighbor and could eventually affect other gas-dependent European nations.

The cutoff intensifies pressure on Ukraine, which is contending with an increasingly violent pro-Russian insurgency in its east and with economic challenges that were crippling even before the upheaval sparked by the ouster of Kremlin-friendly President Viktor Yanukovych in February.

If the gas flow is halted for a long time, winter stockpiles across Europe could be depleted, causing problems across the continent, particularly in eastern European countries that depend solely on Russia for their gas.

Russia’s giant gas company, Gazprom, said Monday that it had notified the European Commission of possible shortfalls following the cutoff. About 15 percent of Europe’s natural gas supplies cross Ukrainian territory.

The two sides appeared Monday to be readying for a protracted battle. The head of Ukraine’s state-owned natural gas company said it could go without new supplies until December, if necessary.

“We will supply gas only in the amount paid for,” Gazprom spokesman Sergey Kupriyanov told reporters Monday in explaining the action against Ukraine. “They paid zero; correspondingly, it’s zero.”

Russian Prime Minister Dmitry Medvedev, meanwhile, called the Ukrainian refusal to pay its gas debts in full “blackmail.”

Ukrainian Prime Minister Arseny Yatsenyuk said Monday he was not interested in accepting higher prices for Russian energy only to allow Russia to “spend this money on weapons, tanks and planes to bomb Ukrainian territory.” The Kremlin has denied arming the rebels.

Russia also halted the import of Ukrainian potatoes Monday, saying they had been found to be contaminated — an action that will hurt Ukraine’s agricultural sector. Russia has frequently restricted food imports from countries with which it is having political disputes.

The cutoff came on a day when Ukrainian President Petro Poroshenko proposed a cease-fire with the pro-Russian separatists but said that the army must first regain control of the porous border with Russia, which he hoped could be done in a week.

In a meeting with his security team, Poroshenko said government troops have reestablished control of stretches of the border where fighters and equipment have slipped across from Russia.

Poroshenko first proposed a truce in his inaugural address when he assumed office on June 7, but rebels rebuffed his call. He also said Monday that he plans to introduce within days constitutional amendments that would give more powers to Ukraine’s regions, a key demand of Russia and the pro-Russian separatists.

Andriy Parubiy, the head of Ukraine’s National Security and Defense Council, estimated that 15,000 to 20,000 “armed terrorists” are in Ukraine’s east — a vast number, if accurate.

He also said that a significant force of well-equipped soldiers has been redeployed close to territories bordering Ukraine in the past day or two. The force’s assets include personnel and transport planes used by the Russian air assault division, he said. There was no immediate response from Russia.

Fierce fighting persisted in Ukraine’s east Monday. In Donetsk, gunmen took over several buildings in the city, including the regional treasury, the national bank and the tax administration. Serhiy Taruta, the governor of Donetsk, said the takeover means the state may have to stop paying salaries and pensions.

The Organization for Security and Cooperation in Europe said as many as 4 million people in the Donetsk region risk losing water because the pumping station and pipeline have been damaged in fighting. The OSCE said city crews started making repairs but stopped because of gunfire nearby. An adviser to Taruta said water could run out within days.

Tensions between Ukraine and Russia are higher than ever after pro-Russian rebels in eastern Ukraine shot down a Ukrainian military transport plane Saturday, killing all 49 people on board. Ukrainian officials accused Russia of supplying the heavy weapons used to shoot down the aircraft.

Protesters in Kiev later attacked Russia’s embassy, overturning cars with diplomatic plates and breaking every window on the front of the building.

Ukrainian and Russian officials have been fighting about gas pricing since Yanukovych was ousted. After Russia annexed the Crimean Peninsula, it hiked gas prices for Ukraine 81 percent, from $269 per 1,000 cubic meters of gas to $485. That price was the highest in Europe, and Ukrainian officials refused to pay, calling it politically-motivated retaliation.

Gazprom has since lowered its price demand to $385, broadly in line with prices for other European countries. Ukrainian officials have sought to pay less and have said the way Russia was structuring the deal meant they would remain vulnerable to price hikes if they did anything to displease the Kremlin.

“Any price they offer is in the form of a discount that can be undone at any time,” said Pierre Noel, an energy security expert at the International Institute for Strategic Studies.

The cutoff is the third in recent years, after previous ones in 2006 and 2009, which also came during disputes over gas payments that Ukrainian and European officials have said were politically motivated.

Ukraine is seeking to soften the blow by buying gas from its neighbors, reversing the flow of pipelines that usually carry Russian gas from Ukraine to them. But such supplies, which would come from Russian sources that reach Europe by another route, are unlikely to satisfy Ukraine’s gas needs.

Underscoring the unlikeliness of a quick resolution, Gazprom and Naftogaz Ukraine filed competing claims Monday in an arbitration court in Stockholm. Gazprom filed to recover a claimed $4.5 billion debt for gas shipments since December. Naftogaz said it has been paying more than the market rate for gas since 2010 and argued that it was owed $6 billion as a result.

Morello reported from Kiev.