U.S. Treasury Secretary Jack Lew said on Sunday he was "encouraged by reports of some progress" in the discussions between Greece and its creditors, urging parties on both sides to show flexibility.

"Rebuilding trust requires demonstrating that a program will be implemented and that there will be measures to make the debt sustainable," Lew said in an emailed statement.

Lew's comments come after a phone call with Greek Prime Minister Alexis Tsipras, the Department of the Treasury said.

(Reporting by Douwe Miedema; Editing by Eric Walsh)