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Military officials participating in trilateral negotiations with Ukraine and Russia and the US made “incremental but significant progress” in determining how a ceasefire would work, a source familiar with the talks told CNN on Wednesday, including finalizing key terms and explanations to help lay the groundwork for future agreements.

While political negotiations remained “tense,” the source said, the military talks left officials feeling cautiously optimistic.

The military-to-military talks over the last two days, held in Geneva alongside political negotiations led by US special envoy Steve Witkoff and Jared Kushner, were led by US Army Secretary Dan Driscoll and Gen. Alex Grynkewich, commander of US European Command.

One of the primary tasks of the military negotiations was getting agreement from both sides on key terms that would be used for the political negotiations, such as the practical terms of a ceasefire and what would constitute a violation of that ceasefire. There was progress made on that front, the source said, though political officials still have to give the final sign-off. Officials expect another meeting to be scheduled in the near future, likely in the next few weeks, the source said.

The political readouts of the talks struck a different tone; Ukrainian President Volodymyr Zelensky said Wednesday that the outcome of the talks was insufficient.

“The military representatives discussed certain issues seriously and substantively,” Zelensky said. “However, sensitive political matters, issues of possible compromises and the necessary meeting of leaders have not yet been sufficiently worked through.”

Presidential aide Vladimir Medinsky, who led the Russian delegation, said the political negotiations were “difficult, but businesslike.”