Standard & Poor's raised Greece's long-term sovereign credit rating on Friday from CCC+ to B- with a stable outlook, the Greek national news agency AMNA reported.

The experts of the credit ratings firm justified the upgrade, citing the country's compliance with the new bailout program in recent months.

"The upgrade reflects our assessment that the Greek government is broadly complying with the terms of its 86 billion euro financial support program financed by eurozone member states via the European Stability Mechanism (ESM)," the agency said in a press release.

Despite differences between the government and its creditors, S&P expects that by April, Greece will meet the conditions attached to the third bailout agreed on in July 2015, therefore the way for discussions on official debt relief will open, the agency's statement said.

"Despite multiple shocks, the economy has proved more resilient than we had previously expected," the report noted.

The development was welcomed as a "positive surprise" by Greek media, as Kathimerini newspaper characteristically commented in a first reaction to the news.

The Greek daily noted that earlier this week the agency had said the new assessment of the Greek economy would be concluded once the new review of the Greek program - which has not started yet - ends.

In addition, the yields of Greek state bonds had increased of late, indicating investors' sentiments, Greek financial analysts noted.