U.S. Secretary of State John Kerry is urging Bulgaria to wean itself from its near-total dependence on Russia for energy and calling for the NATO member to boost anti-corruption efforts.

In meetings with Bulgaria's president, prime minister and foreign minister on Thursday in Sofia, U.S. officials said Kerry was exploring ideas with Bulgarian officials for diversifying their energy supply. Bulgaria relies on Russia for 85 percent of its gas and 100 percent of its nuclear power. The officials added that Russia's recent cancellation of the South Stream pipeline, which would have run under the Black Sea to Bulgaria, is making the need for diversification more acute.

The officials said Kerry is also raising security issues, given current tensions with Russia over Ukraine, and stressing the importance of tackling corruption.