Greece's Prime Minister Alexis Tsipras has offered new concessions to the country's creditors, the BBC News reported.

A letter to creditors sent by Tsipras says he was prepared to accept most conditions that were on the table before talks collapsed and he called a referendum.

On Tuesday, eurozone finance ministers refused to extend the previous bailout.

Germany says talks on a new bailout will not be possible until after the referendum this weekend.

Human rights body the Council of Europe has said the referendum, which will ask Greeks if they want to accept their creditors' proposals, would "fall short of international standards" if held as planned on Sunday.

The body's Secretary General Thorbjorn Jagland said that the fact the vote "has been called on such a short notice... is a major problem", and criticized the lack of clarity in the question to be put to voters.