ATHENS—Greece aims to have an agreement with its international creditors by Sunday, a government spokesman said Thursday, despite warnings from Germany and other creditors that Greece is still far from offering sufficient economic overhauls to unlock bailout cash.

"The optimism expressed by the Greek government is based on actual facts," government spokesman Gabriel Sakellaridis told reporters. "The conditions are ripe to have a deal."

But a European official said Thursday that further progress is needed in the technical talks, reining in expectations from Athens for an imminent deal.

"Open issues need to be resolved," said Annika Breidthardt, a spokeswoman for the European Commission, the EU's executive arm.

Officials in Athens suggested Wednesday that talks with Greece's creditors over fresh bailout funds had reached a final stage, though European Union officials said negotiators still had much work to do before sealing a deal.

Teams from Greece and its international creditors resumed talks in Brussels on Wednesday in a bid to overcome an impasse in their negotiations as Athens runs dangerously low on cash.

"There are no concerns about Greece's banking system," Mr. Sakellaridis said Thursday.

Mr. Tsipras told reporters after a meeting at the Finance Ministry on Wednesday that the two sides were close to an agreement. A senior Greek government official, meanwhile, said Greece and its creditors would start drafting a final list of overhauls and budget cuts that Athens needs to implement to secure fresh bailout funds.

But Valdis Dombrovskis, vice president of the European Commission, played down hopes of a resolution soon, saying Greece and its creditors were still some way from a deal.

"There needs to be a comprehensive reform package presented by the Greek government in order to successfully complete the review," Mr. Dombrovskis told reporters Wednesday.

German Finance Minister Wolfgang Schäuble also said late Wednesday that no progress in substance has been made.

"We always hear positive news coming out of Greece, which is good. However, we haven't gotten much further in substance in the negotiations between the three institutions and the Greek government," he said on German television.

The Greek government hopes that the pessimistic assumptions "do not conceal intentions, apart from estimates," Mr. Sakellaridis said.

Asked whether the Greek government is being blackmailed, Mr. Sakellaridis said, "The government neither receives nor delivers blackmails."

The negotiations between the negotiating teams in Brussels are expected to continue to the end of the week and talks between eurozone finance ministry officials are scheduled for Thursday.

Greece is under pressure to agree to economic overhauls with its creditors to unlock new financing. Athens is believed to have enough cash left to repay a loan of €300 million ($330 million) to the International Monetary Fund on June 5, but probably won't have enough to cover three further repayments due mid-June.

The government has been holding stop-start negotiations with its creditors for weeks, but there is still dissatisfaction with Greece's promises on economic reforms. Significant differences remain on pensions, privatization, labor law and fiscal austerity.

Write to Nektaria Stamouli at nektaria.stamouli@wsj.com

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