Monitors said "little or no gas" was flowing through the pipeline that supplies 80 percent of Russia's natural gas shipments to Europe, The Times of London reported.

With monitors in place and agreements signed, the bickering between Russian and Ukraine continued, The Times said.

A Naftogaz spokesman said the Ukrainian energy company was forced to block gas flows because of "unacceptable transit conditions" imposed by Russia's energy giant Gazprom.

Naftogaz "partially" blocked the flow, which commenced with a televised Gazprom official on the phone ordering the flow to commence.

A technician replied, "I have received the order. We are implementing it."

Russia had shut down deliveries, claiming Ukraine was stealing gas from the pipeline. Previously, Gazprom had reduced deliveries, claiming Ukraine owes $614 million in late fees and would not negotiate its 2009 natural gas contract.

The disrupted gas supplies have left millions in Europe without natural gas supplies for heat during a severe winter chill.

Eighteen countries, including Greece, Italy, Hungary, the Czech Republic, Bulgaria, Slovakia, Serbia and Bosnia have reported supply disruptions.