Greek prime minister Alexis Tsipras has reinforced his diplomatic offensive to try to convince European creditors to pay out the bailout loans the country needs to avoid default.
The creditors have made it clear that Greece has to improve its offer of economic reforms before they release €7.2 billion the country needs to pay debts due at the end of the month.
Mr Tsipras had a short meeting with EU Commission president Jean-Claude Juncker and another with German chancellor Angela Merkel and French president Francois Hollande that lasted into the early hours. He plans to continue talks with Mr Juncker later.
"We decided to intensify the effort to bridge the remaining differences and proceed - I think will proceed - to a solution," Mr Tsipras said after the talks.
The discussions came after the European Commission said the offers made by Greece last week were still not good enough to unlock the bailout funds.
"For this final push, the commission is of the view that the ball is clearly now in the court of the Greek government," commission spokesman Margaritis Schinas said.
With the onus firmly on Greece and options increasingly limited, Standard & Poor's in New York downgraded Greece's credit rating one notch further into junk territory, saying it is likely the country will default on its commercial debt within a year if it cannot strike a deal with creditors.
The ratings agency said Greece has shown it is giving higher priority to its pensions and other domestic spending than making debt payments on time.
S&P lowered Greece's rating to CCC from CCC+ with a "negative" outlook.
Finance ministers from the 19 nations using the euro currency will meet in Luxembourg next week with the Greek deadline for payment ever closer.
The lack of visible progress in the negotiations over past weeks has revived fears Greece could default on its debts and drop out of the euro, a move that would create huge uncertainty for Europe and global markets.
"The goal is to keep Greece in the eurozone," Ms Merkel said. "Where there is a will, there is a way." She insisted it was up to Mr Tsipras first and foremost to show that willingness.
Greece has three weeks to conclude a deal with creditors before its bailout programme expires at the end of the month, when it will also have to repay about €1.6 billion to the International Monetary Fund.
"We must be quick. We must not let things drag out," Mr Hollande said.
Mr Tsipras insisted not only Greece was at risk.
"We must find a solution that will give Greece the opportunity to safely return to growth and a sustainable debt - on a course that will bring back security and stability not just to Greece but all of Europe."