Standard & Poor's on Monday warned it may downgrade the credit ratings of virtually all euro zone countries as the region's debt crisis deepens.

The ratings agency placed the ratings of 15 euro zone countries, including top-rated nations such as Germany and France, on credit watch negative -- an unprecedented move that signals a possible downgrade within three months.

S&P's action was prompted by our belief that systemic stresses in the eurozone have risen in recent weeks to the `=extent that they now put downward pressure on the credit standing of the eurozone as a whole, S&P said in a statement.

S&P expects to conclude its review as soon as possible following the summit of EU leaders later this week, it added.

Of the 17 countries forming the euro zone, Cyprus already had its rating on credit watch negative by S&P and Greece is rated CCC, which already denotes high possibility of default in the near term.

News of S&P's move had leaked to the press in the afternoon, causing U.S. stocks to trim gains and the euro to erase earlier gains.

The other two major ratings agencies, Moody's and Fitch, have already said they could soon review the ratings of top-rated euro zone countries, but they still maintain a stable outlook on top-rated euro zone countries.

(Reporting by Walter Brandimarte and Daniel Bases; Editing by Chizu Nomiyama)