Russia began sending gas to Europe through Ukrainian pipelines on Tuesday, ending nearly a week-long contract standoff that came in the middle of winter.

It will take up to 36 hours for the gas to make its way through the pipelines, according to Reuters.

Ukraine itself is not receiving gas yet due to a contract dispute. Russian officials say differences over how to make sure gash reaches Europe could be a sticking point, Reuters reported.

Monitors are prepared to track the gas shipped through Russia's state owned oil firm OAO Gazprom into European countries through Ukrainian pipelines to ensure it arrives.

Under an agreement signed by Russia, Ukraine and the European Union, the international monitors began to work since Sunday at a main gas pipeline intersection on the Western and Eastern Ukraine borders, the Financial Times reported Monday. Monitors were also positioned at the main gas dispatch center controlled by Ukraine's Naftogaz and another one deployed at Gazprom's main dispatching center, according to officials.

About 75 officials form the monitors group which includes representatives from Gazprom's main European gas buyers: Germany, Italy, France, the Czech Republic, Slovakia, Bulgaria, Greece and Mol-dova, in addition to Russia and Ukraine. Energy Group StatoilHydro from Scandinavia and trade inspector Société Générale de Surveillance will also participate.

Alexei Miller, Gazprom's chief, said Russia was prepared to cut supplies again in case of unsanctioned siphoning of gas from Ukraine. Russia suspended its gas supplies to Europe through Ukraine on Jan. 7 amid allegations that Kiev was diverting them. Ukraine denied the accusations, but the dispute caused Gazprom a reported $800 million, Russian prime minister Vladimir Putin said.