The Greek government cast doubt on EU commission head Jean-Claude Juncker's "sincerity" on Monday, as a bitter war of words erupted over who was to blame for a breakdown in bailout negotiations.

"An essential element in indicating good faith and reliability in negotiations is sincerity," government spokesman Gabriel Sakellaridis said, after Juncker complained he felt betrayed by Athens and urged Prime Minister Alexis Tspiras to tell the Greek people "what is really at stake".