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Petros Giannakouris, Associated Press
Greece's Prime Minister Alexis Tsipras gives a speech during an economic conference in Athens, on Friday, May 15, 2015. Tsipras said that 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 axe pensions and wages.

ATHENS, Greece — Greek Prime Minister Alexis Tsipras late Friday said his country's left-wing government is "very close" to reaching a vital deal with bailout lenders, but insisted there was "no possibility" of giving in to key demands including further cuts to 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 on loan payments.

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

Speaking an annual financial conference in Athens, Tsipras said the government had not abandoned its goal to try and persuade lenders to restructure Greece's debt.

"It appears that we have reached common ground with the institutions on a number of issues, and that makes us optimistic that we are really very close to an agreement," Tsipras said, noting convergence on harmonized sales tax rates and tax administration reforms.

"But several issues remain open ... I want to reassure the Greek people that there is no chance or possibility for the Greek government to retreat on the issue of wages and pensions. Wage earners and pensioners have suffered enough."

The 40-year-old Tsipras won January elections promising undo austerity measures that stabilized public finances but cause a massive drop in output, triggering a wave of poverty and unemployment.

The spat with lenders has already seen Greece slip back into mild recession, and cost the country a troubling drain on bank deposits and a string of ratings downgrades and a troubling drain on bank deposits.

The latest was announced Friday by DBRS, the Toronto, Canada-based ratings agency, which lowered its sovereign rating for Greece from B to CCC with a negative outlook.

"The current downgrade is due to a further increase in uncertainty over whether Greece and its creditors will reach an agreement on a program that restores macroeconomic stability and improves Greece's cash position," the agency said.

"In the absence of an agreement, financing sources appear to be insufficient to meet Greece's financing needs over the foreseeable future."