ATHENS (REUTERS) - Standard & Poor's on Tuesday upgraded Greece's sovereign credit rating by two notches and revised its outlook to stable from negative, citing euro zone countries initial agreement to start negotiations with the country on a third bailout.

S&P raised its rating for Greece to CCC+ from CCC-, saying the country's liquidity perspective has improved after euro zone finance ministers last week gave their initial consent to a three-year loan programme to keep the country in the euro.

"We think opportunities for Greece to default on commercial debt this year are few," S&P said in a report.

The possibly Greece quit the single currency union was lest than 50 per cent, it said.

S&P is the first to upgrade Greece after it clinched a last-minute deal with its lenders last week which opened the way for bridge financing that will cover Athens' funding needs through July and for bailout talks on a third aid package.

The leftist government of Alexis Tsipras tried on Tuesday to rally his Syriza party before a vote in parliament on the second package of measures demanded by international creditors as a condition for opening talks on a new bailout deal.