Greek Prime Minister Alexis Tsipras has written to international creditors saying Greece could accept their last bailout offer if some conditions were changed, but Germany expressed skepticism while saying the door was still open for negotiations.

In exchange, Athens asked for a 29 billion euro loan to cover all its debt service payments due in the next two years.

In the letter, the Greek leader asked to keep a discount on value added tax for Greek islands, stretch out defense spending cuts and delay the phasing out of an income supplement to poorer pensioners.

"As you will note, our amendments are concrete and they fully respect the robustness and credibility of the design of the overall program," Tsipras wrote.

Euro zone finance ministers were due to discuss the Greek request on a conference call on Wednesday, but the initial reaction from ministers and senior officials was that the letter contained elements that ministers would find hard to accept.

German Chancellor Angela Merkel said Greece had not fulfilled its obligations but stressed that the door for negotiations remained open.

Although his letter was dated June 30, it arrived after the 19 Eurogroup ministers had ended a conference call on Tuesday evening. An EU official said it had been received around midnight, when the country's international bailout expired when it defaulted on an IMF repayment.

"The Hellenic Republic is prepared to accept this Staff Level Agreement subject to the following amendments, additions or clarifications, as part of an extension of the expiring EFSF programme and the new ESM Loan Agreement for which a request was submitted today," Tsipras wrote.

However German Finance Minister Wolfgang Schaeuble poured cold water on hopes of a rapid breakthrough, saying the letter had come too late and it was still not clear what Greece wanted.

"That did not provide further clarity," he said, adding that there was "no basis" for serious negotiations with Athens at the moment.

Any talks on a new program would have to start from scratch with different conditions, he told a news conference in Berlin.

Meanwhile, France wants the euro zone to push for a deal with Greece before Sunday's referendum, President Francois Hollande said on Wednesday.

"(A deal) must be found before the referendum, it wouldn't have much sense afterwards," Hollande said. "It is our duty to keep Greece in the euro zone. That depends on Greece ... But it also depends on us. As a European, I don't want the euro zone to come apart, I am not into intransigent comments, into brutal rifts."