SOFIA, Bulgaria — Secretary of State John Kerry said Thursday that the United States would help Bulgaria reduce its dependence on Russia for energy supplies.

Bulgaria, a member of NATO, relies on Russia for 85 percent of the gas it uses and for 100 percent of its nuclear fuel, according to American officials.

“No country in the world should be totally dependent for its energy supply on one other country,” Mr. Kerry, who is making his first trip here as secretary of state, said in a joint news conference with Boiko Borisov, Bulgaria’s prime minister.

“The United States is prepared to help Bulgaria, which has made difficult decisions in order to try to protect its energy future,” Mr. Kerry said. “We are committed to try to help attract investment and provide assistance.”