MOSCOW — Russian state-owned oil giant Rosneft strengthened its hold on the country’s lucrative oil industry when it sealed a deal Monday to buy TNK-BP, the 50-50 joint venture between BP, the British energy country, and a group of Russian oil oligarchs.

The deal will allow Rosneft, already the country’s top oil producer, to increase its global profile. The new combined company will leapfrog ExxonMobil Corp. to become the world’s largest publicly traded producer of oil and gas, in terms of output.