Russian state-run Rosneft oil company and BP have signed a protocol of intention to jointly develop oil and gas fields off of Russia's Sakhalin Island in the Sakhalin 4 project, Rosneft said Wednesday.

"BP will take part in the project under the same terms set for it to participate in the Sakhalin 5 project with Rosneft, that is with a 49 percent stake," a Rosneft spokesman said.

He added that BP would take on the financing of work on the project prior to the start of commercial production.

Sakhalin 4 includes the Astrakhanovsky and Shmidtovsky fields, which are estimated to hold about 120 million tons of oil and 540 billion cubic meters (bcm) of gas.

The Rosneft spokesman said that after signing the protocol of intention, his company and BP would hammer out the details of developing the fields.

BP and Rosneft announced in February that they had agreed to jointly develop the Sakhalin 5 project.

It has estimated reserves of 600 million tons of oil and 600 bcm of gas. Rosneft holds a 51 percent stake in both Sakhalin 4 and 5.