MOSCOW — Russia's natural gas monopoly halted sales to Ukraine in a price dispute Sunday and began reducing pressure in transmission lines that also carry substantial supplies to western Europe.

Ukraine's natural gas company Naftogaz acknowledged the reduction by Russia's Gazprom.

"Gas is not flowing at all through some transit routes, which can lead to a fall in pressure in all the pipelines and limit the overall supply of gas to Ukraine and Europe," Naftogaz spokesman Eduard Zaniuk said. However, he said, "for the people and municipal services there will be enough gas."

Gazprom had given Ukraine a deadline of midnight Saturday to agree to pay quadruple the amount it previously paid for Russian gas, which accounts for about a third of the consumption in the country of 48 million people.

Gazprom supplies about one-quarter of the gas consumed in Europe. Most of that goes through pipes that cross Ukraine and the dispute has raised worries of widespread supply disruptions throughout much of the continent.

"There is information that Ukraine has begun siphoning off Russian gas that is designated for European users," Gazprom spokesman Sergei Kuprianov was quoted by Russian news agencies as saying Sunday evening, about eight hours after Gazprom announced it was stopping deliveries to Ukraine.

Ukrainian Prime Minister Yuriy Yekhanurov hotly denied the allegation, saying "today we are not using a single cubic meter of Russian gas."

Supply problems to Europe could undermine Western trust in Russia's natural gas industry, one of the keystones of the country's economy, and tarnish Russia's stint as chairman of the Group of Eight, which formally started Sunday.

A Foreign Ministry statement said Russia would "strictly fulfill" its supply commitments to Europe and that "responsibility for any possible ... problems for European countries caused by the actions of Kiev will lie with Ukraine."

The supply restrictions were felt immediately. In Hungary, which takes Russian gas that has flowed through pipes in Ukraine, major power users were asked to switch to using oil as Russian gas supplies fell by 25 percent.

The only gas being put into pipelines headed for Ukraine is intended for European customers, according to Gazprom.

The showdown has underlined the tensions between the two former Soviet republics since Ukrainian President Viktor Yushchenko — a West-leaning leader who wants to reduce Moscow's clout in his country — defeated a Russian-backed rival in a bitter electoral battle a year ago.