Russia's state-owned Rosneft said Saturday it had found oil in the Kara Sea off the north coast of Siberia in a joint drilling project with US oil giant ExxonMobil.

The announcement comes after the US targeted Rosneft, Russia's largest oil company, and its chief Igor Sechin with sanctions over Moscow's role in the Ukraine conflict.

"Oil has been discovered" in the University-1 well, Russia's northernmost, Rosneft said in a statement.

Sechin, a close ally of President Vladimir Putin, said at the opening ceremony that the discovery was a "joint victory," and thanked "friends and partners" including ExxonMobil, quoted in the company statement.

The oilfield will be named Victory, Sechin said. In a sign of the importance of the exploration project to Russia, Putin took part in the launch ceremony in August via a video link to the West Alpha oil rig.

ExxonMobil said earlier this month that it would "wind down" the project following US sanctions.

The Treasury Department gave US companies until September 26 to bring to an end exploration or production ventures with Rosneft and four other Russian energy companies on deepwater, Arctic offshore or shale projects.

Rosneft said Saturday it is currently exploring three areas in the Kara Sea that are believed to contain 87 billion barrels of oil. It said the Kara area's total reserves could rival those of Saudi Arabia.

The Arctic zone has extreme conditions, requiring advanced technology and massive investment.

Oil drilling in the pristine region is opposed by environmentalist campaign groups including Greenpeace, whose activists were jailed last year after trying to board a drilling platform belonging to the Gazprom energy giant.