"The situation is extremely difficult, extremely tense. The result of the elections showed a very strong reversal for the two governing parties and an increase of extremes, so it is extremely worrying," he told Europe 1 radio.
Greece is facing a political impasse after the vote Sunday pushed out the parties accepting an EU-IMF bailout package that included harsh austerity measures.
After the conservative New Democracy party failed to form a government Monday, the task has fallen to radical left-wing party Syriza – whose leader, Alexis Tsipras, said Tuesday the vote had "clearly nullified" Greece's loan agreement.
"The questioning of the treaties that were painstakingly negotiated ... risks sparking turbulence that will be difficult to control," Juppé said.
Juppe was to attend his final cabinet session as foreign minister later Wednesday after President Nicolas Sarkozy was ousted by Socialist François Hollande in France's election on Sunday.
Hollande has vowed to renegotiate the EU's fiscal pact to put emphasis on economic growth as well as austerity but is facing fierce resistance from Germany.