By Jeff Mason and Steve Holland WASHINGTON (Reuters) -Russia now has enough forces to conduct a major military operation against Ukraine and an assault aimed at seizing large parts of that country could begin “any day now,” White House national security adviser Jake Sullivan said on Friday. Sullivan, speaking at a White House briefing without listing specific evidence, said any American still in Ukraine should leave in the next 24-48 hours. He said a Russian invasion could start with an air assault that would make departures difficult. Sullivan said U.S. intelligence believes Russian President…