It is the first official visit to the eastern European nation for the top US diplomat since he came to office almost two years ago.
US Secretary of State John Kerry arrived in Bulgaria Thursday seeking to help the cash-strapped country wean itself off Russian energy supplies and battle rampant corruption.
"Bulgaria is an ally that is in need of strong support at the moment. The economy has been somewhat sluggish. They have significant energy dependence problems," a senior State Department official told reporters.
In his talks with Bulgarian leaders, Kerry will also compare notes after Sofia arrested a Frenchman on January 1 who apparently had links to one of the two brothers behind last week's deadly attack on the Charlie Hebdo satirical weekly in Paris.
Bulgaria's need for energy diversification has become pressing after the EU member in June froze all work on its section of a pipeline project called South Stream, under heavy pressure from the European Union and the United States.
The pipeline was intended to bring 63 billion cubic metres of Russian gas underneath the Black Sea and through the Balkans, crossing Bulgaria, Serbia, Hungary and Slovenia and then Austria to connect with the main European pipeline network -- and bypassing crisis-hit Ukraine.
But amid a downturn in Russian relations with the EU and the US due to the Ukraine conflict, Moscow pulled the plug on the pipeline project in December.
Sofia relies on Russia for 85 percent of its gas, and 100 percent of its nuclear-generated electricity.
"It is not a good position for a NATO ally to be (in) -– and to be 85 percent dependent on one country for gas," the US official said, asking not to be named.
"So this is part and parcel of the larger US policy to support energy diversification across NATO and EU space."
Plans to build a spur for Bulgaria to a Liquid Natural Gas (LNG) plant in Greece would be part of Kerry's discussions with Bulgarian President Rosen Plevneliev and Prime Minister Boyko Borisov.
He will also raise delays in a $5 billion project agreed last year with US nuclear engineering giant Westinghouse to build a new reactor at the country's Kozloduy nuclear power plant.
"There have been a number of problems that have to do with the economic support for the deal -– whether that exists, et cetera, how much we can do in terms of outside support," the US official said.
The US diplomat will also meet with his British counterpart Philip Hammond who is on a tour of Bucharest and Sofia.
Corruption, poverty woes
Washington was also looking "to see more commitment to fighting corruption" from Borisov's government which came together after October elections.
It is the country's third government in two years and comes as it needs to improve the sluggish economy, implement unpopular reforms and get to grips with rampant corruption, cronyism and organised crime.
The average monthly salary in this southeastern European country of 7.4 million is the equivalent of 400 euros ($500), and seven years after joining the EU every fifth household lives below the poverty line.
Undermining faith in democracy in ex-communist Bulgaria is the perception that a well-connected clique are still lining their pockets and are above the law.
Anger at such corruption and poverty erupted into nationwide protests that saw several people die after setting themselves on fire in 2013. Last month four people self-immolated themselves in similar protests.
"Corruption is not just a threat to the democratic fabric of a country, it's not just something that undercuts the environment for investment, it also is a wormhole for foreign interference," the US official said.