Russia's top natural gas producer Gazprom said an intergovernmental agreement with Beijing to seal a US$400 billion deal to supply gas to China through an eastern route had yet to be signed, clouding the plan's prospects. Gazprom said, however, the final document to be signed by the Russian and China governments was almost ready, and could be signed "in the nearest future". Gazprom and Russia and China National Petroleum Corp signed the deal to supply gas through a pipeline in May after a decade of painstaking talks. Russian President Vladimir Putin lauded the agreement as the biggest contract in the "history of the gas sector of the former USSR". The deal now needs the signature of both governments. According to a Russian government document, the agreement between the Chinese and Russian governments is a condition under which the deal, signed in May, can come into force. Some analysts and insiders have already expressed their doubts over the validity of the deal, saying that a final agreement on price had not been reached. Russia has announced gas deals with China several times, only for them to prove elusive. In 1997, for example, a US$7 billion deal was clinched to supply 25 billion cubic metres of gas a year to China but the project never materialised. The latest absence of an agreement brings uncertainty to Moscow's plans to diversify its energy exports to the east from the west. Russia has been stepping up the plans due to the crisis in Ukraine, which has caused the deepest rift with Europe and US since the end of the cold war.