Greece said Thursday it aims to clinch a deal with its creditors by Sunday that will allow it to receive the desperately needed final installment of its international bailout plan and keep it from defaulting on its debts.

"This optimism that the Greek government is expressing is not idle talk. It is based on very specific facts," government spokesman Gabriel Sakellaridis told reporters at a briefing. "We are going into these negotiations with the aim to have an agreement with our partners by Sunday."

Sakellaridis' comments come a day after Prime Minister Alexis Tsipras expressed similar optimism that a deal was near, and Greek officials said the text of an agreement was being written up. The optimism, which caused a market rally late Wednesday, proved short-lived, however, after key creditor states like Germany warned a final agreement remained elusive.

Greece's new government, elected in January, has been in talks for four months on what reforms it should make to get the final bailout installment of 7.2 billion euros ($8 billion) from fellow eurozone states and the International Monetary Fund.

The radical left Syriza party won the elections on promises to repeal deeply resented austerity measures that accompanied the country's bailout, arguing they had simply increased hardship and poverty and left the economy in a depression.

But Greece's creditors insist it must take measures to ensure its economy is reformed and doesn't slip back into the bad habits of the past with reckless borrowing and an overinflated an inefficient public sector. Negotiations on what measures exactly Athens must take have dragged with little evidence of progress for months.

"We believe conditions are now ripe for an agreement to be reached. Many discussions have been made, the Greek government has submitted very specific proposals, it has submitted very realistic proposals," Sakellaridis said.

"A constructive discussion is being held, and all this inspires us with optimism that in this short period (of time) there will be a positive outcome and agreement."

Unable to tap the international borrowing market due to painfully high interest rates demanded for its bonds, Greece cannot continue repaying its debts without the last bailout installment, and risks a default that could eventually lead it out of the eurozone.

Athens must repay just over 300 million euros to the IMF on June 5.

The installment is just one of four due to the IMF in June, all repayments of initial loans that made up Greece's 240 billion euro bailout, which began five years ago.