Sofia and Bucharest, both Nato newcomers, have offered a number of sites to serve as a possible hub for US forces as Washington shifts its stance from large Cold War installations in central Europe and Asia to a smaller, more mobile format.
Negotiations are nearing a close with Romania for the use of at least one base near its Black Sea coast. In Bulgaria, talks are centred around two sites - the Bezmer airfield and the Novo Selo firing range - both near its southern border with Turkey.
"Romania may be wrapped up by the end of the year and I would expect Bulgaria to be concluded sometime after that," US ambassador to Bulgaria John Beyrle told Reuters.
He declined to give further details on the talks but said the proposed bases could host several thousand US troops during training operations on short rotations and a small group of permanent administrative staff.
Bulgarian officials have said 2,000-3,000 troops may use the sites at a time, although both sides have made clear they will not resemble the permanent bases that housed tens of thousands of US troops in central Europe during the Cold War.