"This unexpected decline was only possible because of grave errors by BP, which leads the consortium that operates the Azeri and Chirag fields," Aliyev told a meeting of Azerbaijan's cabinet late Wednesday.

"Azerbaijan lost out on $8.1 billion in revenues" because of the lower than expected output, he alleged.

Aliyev accused the British oil giant of not fulfilling its promises about output levels and said "serious measures" must be taken to rectify the situation.

"Inaccurate forecasts are unacceptable to us. False pledges given to (Azerbaijan's) state oil company are unacceptable," he said in the comments to his cabinet that were televised on Thursday.

BP-Azerbaijan said it was working to resolve any problems.

"The company is fully loyal to Azerbaijan and is working with the State Oil Company of Azerbaijan (SOCAR) to address issues of production in the Azeri-Chirag-Guneshli fields in the shortest possible time," BP-Azerbaijan said in a statement Thursday.

BP has considerable assets in Azerbaijan, including stakes in the Azeri-Chirag-Guneshli offshore oil fields and the huge Shah Deniz offshore gas field.

It also part-owns and operates the strategic Baku-Tbilisi-Ceyhan pipeline, the main export route for Azerbaijani oil.

A mainly Muslim country wedged between Russia and Iran, Azerbaijan is a key partner in projects to deliver Caspian Sea energy reserves to the West through oil and gas pipelines to Turkey, bypassing Russia.