Russia's state-controlled gas monopoly Gazprom said it began pumping gas to Europe at 10 a.m. (0700 GMT, 2 a.m. EST), ending a six-day cutoff, but four hours later Gazprom's Deputy Chairman Alexander Medvedev said Ukraine's pipeline system had failed to carry it on to Europe.

"Ukraine didn't open any export pipelines," he told reporters. "They just shut down the entry of the pipeline in the direction of the Balkans. We don't have the physical opportunity to pump the gas to European customers."

Underscoring political tensions behind the gas dispute, Medvedev accused Washington of encouraging Ukraine's defiance. "It looks like they are dancing under the music that is orchestrated not in Ukraine," he said Tuesday.

Ukrainian energy adviser Bohdan Sokolovsky said Russia deliberately shipped the gas along a technically arduous route that requires Ukraine to cut domestic consumers out before it can deliver gas to the Balkans. He said a gas entry point on the Russian border and a gas pumping station near the Romanian border where Gazprom wants its gas delivered are not linked by an export pipeline.

"They are continuing their campaign to discredit Ukraine," Sokolovsky told The Associated Press. Ukraine's President Viktor Yushchenko accused Russia of using the gas dispute to try to wrest control of Ukraine's 23,000-mile (37,000-kilometer) gas pipeline network.

EU spokesman Ferran Tarradellas Espuny, meanwhile, said both nations had deprived EU monitors of full access to their natural gas control rooms. "Access to the dispatching rooms is essential to know what is actually happening," he said, adding it's "too early to draw such conclusions" on who to blame.

EU Commission President Jose Manuel Barroso called Russian Prime Minister Vladimir Putin to express "disappointment" over the lack of gas flow to Europe and Russia's failure to let monitors to the control room.

Barroso's aide said Putin promised to look into both  matters. Putin's office said he told Barroso that Ukraine is to blame for the problem and he should call them.

Putin also spoke to his Bulgarian and Slovak counterparts, urging them to increase the pressure on Ukraine to ensure gas flow to Europe. "The information that we have from our monitors in Russia is that little or no gas is currently flowing and we are not at this stage jumping to conclusions as to why this is the case," said another EU spokeswoman, Pia Ahrenkilde

Hansen. "This situation is obviously very serious and needs to improve rapidly." Russia supplies about one-quarter of the EU's natural gas, 80 percent of it shipped through Ukraine's vast pipeline network. Amid a pricing dispute with Ukraine, Russia cut off gas supplies to Europe on Jan. 7 just as the continent was gripped by freezing temperatures.

Russia has accused Ukraine of stealing gas intended for Europe and only restarted supplies after a EU-led monitoring mission was deployed to gas metering and compressor stations across Ukrainian territory. Ukraine has denied the charges, claiming that Russia has not sent enough gas to pump the rest of it west to Europe.

The two nations are deeply at odds over who should pay for this so-called "technical gas" _ gas Ukraine needs to power the compressors _ and the amount of gas needed is substantial. Ukraine warned Tuesday it will have to use some gas from Russia for that purpose but Russia said it would consider that theft.

The gas cutoff has affected more than 15 countries, with Bosnia, Bulgaria, the Czech Republic, Hungary, Serbia and Slovakia among the worst hit. Sales of electric heaters have soared and thousands of businesses in eastern Europe have been forced to cut production or even shut down.

Millions of people have been affected by the heating crisis or involuntary layoffs. Russia stopped gas supplies to Ukraine itself on Jan. 1. Russia has used the gas dispute to push for prospective gas pipelines under the Baltic and the Black Sea that would bypass Ukraine. But EU officials say the crisis should encourage a search for independent energy sources and supply routes, such as the U.S.-backed Nabucco pipeline via Turkey that would carry Caspian energy to Europe and circumvent Russia.

Masha Lipman of the Carnegie Endowment's Moscow office said the dispute will push the EU toward finally "creating a European gas market" that would reduce Russia's clout as an energy supplier. The EU will also have to reconsider the options of nuclear and coal-fired plants, she said.

Relations between the two ex-Soviet neighbors have been strained since the 2004 Orange Revolution in Ukraine led to the election of a pro-Western government in Kiev. Ukraine's efforts to join NATO and its support for the former Soviet republic of Georgia in its August war with Russia also angered the Kremlin.

Russia still will not send natural gas to Ukraine for domestic consumption until the deadlock is resolved over what Ukraine should pay for Russian gas in 2009 and what Russia should pay for using Ukraine's pipelines.

Ukraine last year paid $179.50 per 1,000 cubic meters of gas and Yushchenko said Tuesday that Ukraine will pay no more than $210 in 2009. Russia wants Ukraine to pay market price for gas, about the $450 that European customers pay.

Associated Press writers Maria Danilova and Yuras Karmanau in Kiev, Ukraine, Nataliya Vasilyeva and Douglas Birch in Moscow, and Raf Casert in Brussels contributed to this report.