File Photo: A Bulgarian transport aircraft C-27J Spartan and Bulgarian paratroopers participate in the joint Bulgarian-U.S. military exercise at Krumovo Airbase, near Plovdiv, Bulgaria, on July 16, 2015. (Xinhua/Wang Xinran)

WASHINGTON, Dec. 19 (Xinhua) -- U.S. Secretary of State Mike Pompeo said the United States would continue to strengthen security relations with Bulgaria, according to a statement issued by the State Department on Wednesday.

Pompeo had a phone conversation with Bulgarian Prime Minister Boyko Borissov on Tuesday, emphasizing that Washington would commit to working with Sofia to tailor the final scope of a potential F-16 fighter aircraft sale to fit the budgetary and operational requirements of Bulgaria, said the statement.

The United States had provided over 200 million U.S. dollars in security assistance to Bulgaria over the last 25 years, and would continue to prioritize investments with Bulgaria that build NATO interoperability, Pompeo said in the statement.

Bulgaria, a NATO member since 2004, has been planning to replace its aging Russian made MiG-29 fleet with new multi-role combat aircraft, according to the Military Balance 2018 issued by the International Institute for Strategic Studies, a London-based think tank.

Moscow has regularly accused NATO of boosting its military presence in Eastern Europe, including increasing military contingents in the Baltic states, Poland, Romania and Bulgaria using fears for possible Russian aggression as a pretext.