The United States embassy in Bulgaria has warned its citizens to avoid a busy transport hub in the capital Sofia because of a potential threat against an unspecified bus line.

"The US Embassy has received information of a possible threat against an unspecified bus line or bus lines in the vicinity of Hotel Pliska," the embassy said in a statement posted on Facebook late on Wednesday, advising its citizens to avoid the area, on one of Sofia's main boulevards.

A spokeswoman confirmed the authenticity of the statement but gave no further details, saying the embassy did not comment on pending investigations.

The interior ministry said it was not aware of any credible threat in the vicinity, known informally for years as Hotel Pliska, but Sofia Mayor Yordanka Fandakova said police had stepped up their presence.

People appeared to be going about their business normally in the area, with buses running according to their schedule.  - Reuters