BERLIN—The Russian and Ukrainian foreign ministers on Wednesday pledged to work to reach an "unconditional and mutually agreed sustainable cease-fire" in Ukraine by the end of the week as violence in the country's east continued to flare.

The declaration adopted by the diplomats, along with the German and French foreign ministers, after all four met in Berlin on Wednesday, didn't mention a new unilateral cease-fire by Kiev that Russia has been calling for. Instead, the declaration said that a "contact group"...