"The next step is clear: full implementation of the agreement -- no more, no less," Juncker told Russia's main economic forum in Saint Petersburg ahead of a meeting with President Vladimir Putin.
"This is the only way to begin our conversation and the only way to lift the economic sanctions that have been imposed."
During the meeting, part of which was broadcast on Russian state television, Juncker told Putin that "some in Europe weren't pleased" with him meeting the Kremlin strongman but stressed it was an opportunity to "exchange opinions".
Putin is hosting Juncker as well as Italian Prime Minister Matteo Renzi at the three-day conference in Russia's second city just weeks before the expiry of EU sanctions that have helped plunge Russia's economy into recession.
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Putin will also sit down with UN chief Ban Ki-moon alongside the world body's envoy on Syria Staffan de Mistura, as he seeks to recast Moscow as a more stable partner for the West and bolster his country's flagging economy.
But the accord has stalled as the violence -- that Kiev and the West says is masterminded by Moscow -- rumbles on.
And despite some signs of cracks in the EU over extending the sanctions, Juncker insisted the bloc was "united".
Diplomatic sources in Brussels told AFP that EU members could agree to renew the economic sanctions against Russia for six months as early as next week as they look to clear the schedule ahead of a summit at the month's end that is likely to be dominated by Britain's EU referendum.
"If our relationship today is troubled and marked by mistrust, it is not broken beyond repair. We need to mend it, and I believe we can," he said.