Russia's Foreign Ministry has called "unacceptable" Bulgarian media reports of behind-the-scenes efforts to pressure Sofia into changing its UN Secretary General candidate.

Attempts to exert influence on countries' decisions to nominate one or another candidate are "unacceptable", TASS quotes the ministry's spokersperson Maria Zakharova as saying.

She has also argued the reports were merely attempts to embroil Russia in political games.

Government sources said on Saturday Bulgaria might resort to a new nomination for UN chief, amid reports several Eastern European countries, supported by Germany, were planning to raise EU Commission Vice President Kristalina Georgieva's nominaiton.

Some media outlets suggested German Chancellor Angela Merkel had talked Russian President Vladimir Putin into backing Georgieva. Bokova, whose performance at the Security Council straw polls questions her chances to win, has enjoyed stable support from Moscow.

Zakharova on Sunday said any information that Russia might be supporting such a move were "a lie".

However, she argued Germany had attempted to discuss the issue.

"[Germany's] Chancellor tried to begin a conversation with the leadership of Russia with regard to the possible support from [Russia] for a candidate from Bulgaria other than the one officially nominated."

At the G-20 summit, "Ms Merkel was clearly and clearly and unambiguously told that the appointment of candidates to the position of UN Secretary General is exclusively and only a sovereign decision of the country."