KIEV, Ukraine—U.S. Vice President Joe Biden warned Russia of further isolation should the Kremlin fail to encourage peace in eastern Ukraine, but he stopped short of announcing lethal military aid for Kiev.

Mr. Biden’s visited the Ukrainian capital on the anniversary of the country’s pro-Europe protest movement. The trip came as five Ukrainian political parties, including President Petro Poroshenko’s faction, on Friday signed an accord to form a coalition in the Ukrainian parliament nearly a month after the pro-Western parties...