Russia's Gazprom said it could restart gas shipments to Europe on Friday if an agreement can be signed allowing an EU-led monitoring mission to track gas flows through Ukrainian pipelines.

European Union monitors began working Friday in the Ukrainian capital of Kiev, but Gazprom CEO Alexei Miller said a final agreement on their deployment has yet to be signed. Russia wants monitors in place to prevent what it described as Ukraine's theft of supplies meant for Europe — a charge Kiev hotly denies.

"Our goal is to show who is to blame for stealing gas," Russian President Dmitry Medvedev said. "Such thievery can't be left unaccountable."

Gazprom halted all natural gas shipments through Ukraine on Wednesday, ending or reducing gas supplies to more than a dozen European nations amid a pricing dispute with Kiev.

Click here for photos.

Russia in the past has sold gas to Ukraine and some other ex-Soviet neighbors at prices significantly less than European prices.

Medvedev said Friday that Ukraine should pay a European price for the Russian gas. Last year, Russia charged Ukraine $179.50 per 1,000 cubic meters, about half what it charged its European customers.

Russia's last offer before talks broke down was $250, but Gazprom said the offer no longer stands after Ukraine rejected it and that it will charge Ukraine $450.

EU governments have criticized both Russia and Ukraine for the gas crisis, saying it was unacceptable to see homes unheated, businesses closed and schools shut down in the middle of winter because of the commercial squabble.

Miller pledged Gazprom would resume shipments to Europe once the EU and Russian monitors were in place at pipeline pumping stations across Ukraine — a country roughly the size of South Africa or Texas. The EU said it could then take days for the shipments to reach western Europe.

The EU observers visited the central dispatching point of Ukraine's gas pipeline gas operator, Ukrtransgas in Kiev.

"This means that all the conditions agreed between the leaders of the European Union, Russia and Ukraine are in place for the immediate restart of gas supplies from Russia that are destined for European customers," the EU Commission said in a statement.

But Medvedev, who met with Miller in the southern Russian city of Sochi, emphasized that Russia will only resume deliveries to Europe after a written agreement is in place.

Miller later said that Ukraine had softened its position and expressed hope for signing the deal quickly.

Ukraine's state company Naftogaz spokesman Valentyn Zemlyansky said Russia was the one dragging its feet on the deal.

"They are just wasting time, trying to make this process as long as possible," Zemlyansky told The Associated Press.

Czech Prime Minister Mirek Topolanek, whose country holds the EU presidency, was traveling in Kiev to help wrap up the monitoring deal.

"The mission is sensitive. Both sides want to save face," Topolanek said before departing from Prague. "I'm a bit optimistic, but don't expect me to give any assurance that it will be definitely solved today."

Russia had insisted its representatives be included in the EU monitoring mission, and Ukraine agreed on Friday, officials from the EU and Ukraine said.

"It is now imperative that the gas starts to flow," the EU said in a statement.

Once gas shipments resume, it "will take at least three days" for the first gas to reach European consumers, EU spokesman Ferran Terradellas said.

The halt in gas supplies has left European nations struggling to cope during a harsh winter. At least 11 people have frozen to death this week in Europe, including 10 in Poland, where temperatures have sunk to minus 13 F.

Fifteen countries — Austria, Bulgaria, Bosnia, Croatia, the Czech Republic, France, Greece, Hungary, Italy, Macedonia, Romania, Serbia, Slovakia, Slovenia and Turkey — said their Russian supplies ceased Wednesday. Germany and Poland also reported substantial drops in supplies.

French Prime Minister Francois Fillon urged a unified European response "so as not to be held hostage to the Russian-Ukrainian energy crisis."

"I solemnly call on the two parties to scrupulously respect their obligations with us," he told a capitalism conference in Paris.

Ukraine's Naftogaz promised the first gas supplies would go to Bulgaria, where thousands of homes are without heating and factories have been shut.

The Sofia Zoo in the Bulgarian capital declared an emergency Friday after being left with no central heating. The zoo was using electric heaters for its 1,300 animals, some of which needed temperatures of at least 68 F, director Ivan Ivanov said.