Greek debt swap negotiators were less optimistic on Friday about reaching an agreement to avert a disorderly default after the latest round of talks with creditor banks, warning failure to reach a deal would be disastrous for Greece and Europe.

Yesterday we were cautious and confident. Today we are less optimistic, said a source close to the Greek task force team in charge of negotiations.

It is important to remind all parties that the consequences of failure would be catastrophic for Greece and the Greek people, Europe and Europeans, the source said on condition of anonymity.

Charles Dallara, who held meetings in Athens on Thursday and Friday on behalf of the Institute of International Finance representing creditors, left without agreement on a deal that would effectively see banks voluntarily give up a lot of their promised returns.

There is a meeting next week and we'll make every effort to succeed, the source added.

(Reporting by Dina Kyriakidou)