Greece has made huge progress by returning to economic growth, former U.S. Treasury Secretary Jack Lew told state broadcaster ERT in an interview.

"If you think of where Greece was during the crisis, it has made huge progress by growing again, that it is achieving difficult fiscal targets - for some people unachievable - while the drop in unemployment is a very good sign," said Lew, who was at the helm of the US Treasury during the peak years of the Greek crisis.

He described the completion of the third bailout as a very big achievement, adding that he believes a growth period will follow but warned against the reversing of reforms.

"I believe that the challenge as Greece moves ahead is not to go back on reforms that produce essential results, not to go back and create large fiscal weights, such as in the years of the crisis, not to be driven to excessive fiscal adjustments, not to spend money that it does not have and not to harm the progress that it has already achieved," he said.