(Adds details on immigration cluster, details throughout) TOKYO, Feb 19 (Reuters) - Japan confirmed a new variant of COVID-19, and an infection cluster emerged at a Tokyo immigration facility, presenting new challenges as the country tries to overcome a third wave of the pandemic. The new variant has been found in 91 cases in the Kanto area of eastern Japan and in 2 cases at airports, Chief Cabinet Secretary Katsunobu Kato told reporters on Friday. The government is raising surveillance against mutant varieties as they may be more resistant to vaccines, which Japan started to distribute this week. "It may be more contagious than conventional strains, and if it continues to spread domestically, it could lead to a rapid rise in cases," Kato said.