WASHINGTON (Reuters) - The United States does not believe a Russian amphibious assault in or near the Ukrainian city of Odessa is imminent, a senior U.S. defense official said on Sunday, amid growing concern about a potential attack on the city.

       The official, who spoke on condition of anonymity, estimated Russia had launched about 600 missiles since the start of the invasion and deployed into Ukraine about 95% of combat forces it had pre-staged outside of the country.

       Russian forces continued to try to advance and isolate Kyiv, Kharkhiv and Chernihiv and are meeting "strong Ukrainian resistance," the official said.

       (Reporting by Idrees Ali and Phil Stewart; Editing by Chris Reese)