U.S. Secretary of State Hillary Clinton's visit to Sofia on Sunday will be warmly welcomed by Bulgaria but likely be shadowed by the Bulgarian energy and military dependence on Russia.

During her one-day visit, Clinton will meet with senior Bulgarian officials on issues including the democratic transitions in the Middle East, the U.S. ongoing support to Afghanistan and energy security, said the U.S. Department of State.

"Bulgaria is a good friend and loyal partner of the United States, and the Secretary of State will visit your country as a token of our appreciation," U.S. ambassador to Sofia James B. Warlick said on Tuesday.

Although a U.S. rival during the Cold War, Bulgaria gradually became one of the most faithful and predictable political allies of the overseas giant, supporting its initiatives in key areas such as Yugoslavia, Iraq and Afghanistan, providing its airspace or troops.

In 2006, the United States gained military access to four Bulgarian military facilities, including two air bases. In 2011, Bulgaria said it was ready to host any elements of the future NATO missile defense system.

Analysts believe there is no doubt that Bulgaria will respond positively to Clinton's foreign policy, even if they are related to the issues in Iran or Syria.

Perhaps, Clinton will also praise the friendly government of Boyko Borissov in its fight against crime, amid Bulgaria's failure to join the visa-free Schengen area.

Following this direction, the United States could go even further, dropping the visa requirement for Bulgarians during this visit, or promising to do so. A hint of such a move was made by the Bulgarian Foreign Minister Nickolay Mladenov on Sunday that the visa issue would be discussed during Clinton's visit.

However, Clinton's talks with Bulgarian officials in Sofia are likely to be shadowed by the Bulgarian energy and military dependence on Russia.

The United States have constantly expressed their concerns about Bulgaria's energy dependence on Russia.

Although Borissov has frozen the project for construction of the Belene nuclear power plant won in 2006 by Russia, Bulgaria, a former close friend of the former Soviet Union, still relies on Russia for 96 percent of its gas needs.

On top of that, in January, the Bulgarian government prohibited the U.S. Chevron Corporation to make oil and gas exploration in the country by using hydraulic fracturing, changing the company's permit issued last June, where no technology was specified.

Clinton will probably also express dismay over recent statements of Borissov that Bulgaria had postponed purchase of new NATO-compatible multi-role fighters for an indefinite future because of other government spending priorities.

The problem is that the Soviet-made MiG-21 and MiG-29 fighters are the only fighters in the Bulgarian air forces, and a tender for their replacement with Western weapons such as a squadron of secondhand Lockheed Martin's F-16 Fighting Falcon has been under preparation since Bulgaria joined NATO in 2004.

"We will buy new fighters, only when it becomes extremely indispensable because of our membership in NATO," Borissov told reporters last September.

Perhaps the only key unpredictable result of the Clinton's visit to Sofia is whether she will convince Borissov that the time for retirement of the Soviet fighters in Bulgaria has come.