SAINT-PETERSBURG - Russian oil giant Rosneft and Chinese state firm CNPC on Friday signed a $270 billion deal to supply China with oil over 25 years, an agreement hailed by Russian President Vladimir Putin as "unprecedented."

The agreement was signed by Rosneft chief executive Igor Sechin and CNPC head Zhou Jiping in the presence of Putin and visiting Chinese Vice Premier Zhang Gaoli.

"An estimated value of the contract in current market parameters is absolutely unprecedented — 270 billion dollars," Putin told participants of the annual Saint Petersburg International Economic Forum.

Addressing chiefs of global energy companies like Eni, ExxonMobil (IW 500/1) and Statoil, Putin reiterated Russia's plan to diversify its energy clients away from its traditional reliance on Europe.

Putin has made a priority of stabilizing Russia's sometimes prickly relations with its giant eastern neighbor at a time when its ties with the West are becoming ever more problematic.

Russia wants to diversify its base of energy customers away from Europe and is aware it has not fully exploited the colossal potential of the Chinese market.

"Essentially, this is a new era of cooperation which means that in our cooperation with our strategic partners we shift from purely raw supplies to full-fledged cooperation in the engineering and manufacturing sphere," Putin said.

Top Putin ally Igor Sechin told reporters earlier that the deal would involve the delivery of more than 360 million tons of oil over 25 years with a total value of $270 billion (204 billion euros).

Russia is slated to receive an upfront payment of some $60 billion. Sechin said that deliveries could start as early as this year.