Fitch cut Greece's credit rating to junk on Friday, highlighting persisting doubts over the country's ability to pull itself out of a severe debt crisis that has shaken the euro zone.

The rating agency cut Greece's rating to BB-plus from BBB-negative and kept it under negative outlook.

Greek debt is now rated junk by all three major rating agencies.

"The downgrade acknowledges that while Greece's economic and fiscal performance under the EU-IMF program has in many respects exceeded expectations, its heavy public debt burden renders fiscal solvency highly vulnerable to adverse shocks," Fitch said in a statement.

The Greek finance ministry said the downgrade showed the need to revise how rating agencies operate in Europe.