Greek Prime Minister Alexis Tsipras says his country's left-wing government is "very close" to reaching a vital deal with bailout lenders, but insists there is "no possibility of retreat" on demands to further ax pensions and wages.

Tsipras, who marked 100 days in office this week, is struggling to hammer out a deal with lenders that would release final payouts of the current bailout agreement, seen as crucial to avoid a summer default.

Eurozone creditors and the International Monetary Fund argue that Athens' proposals are still too vague.

The spat has already cost Greece a string of ratings downgrades and a troubling drain on bank deposits.