PISAREVKA, Russia — Russian natural gas began flowing Tuesday into Ukraine after a nearly two-week cutoff that left large parts of Europe cold and dark and underscored the continent's vulnerability and dependence on Russia's energy.

Russia's gas monopoly Gazprom began pumping gas into Ukraine at around 10:30 a.m. Moscow time, spokesman Boris Sapozhnikov said by telephone from the Sudzha metering station on the border with Ukraine. Ukraine's Naftogaz state gas company confirmed gas was being received.

Naftogaz said the shipments could take up to 36 hours to cross Ukraine, which is the size of France, and reach European customers. Europe receives about a fifth of its natural gas from Russia.

Russia halted the supplies on Jan. 7 as it argued with Ukraine argued over 2009 gas prices and allegations that Ukraine was stealing gas destined for Europe.

More than 15 nations in the Balkans and Eastern Europe were left scrambling for alternative energy sources, with factories shut down and millions of people shivering in unheated homes. Bulgaria and Slovakia, in particular, rely almost entirely on Russia for gas.