A top White House official on Tuesday urged state governers to work with the U.S. Food and Drug Administration to extend the shelf life of Johnson & Johnson's COVID-19 shot as millions of doses nationwide sit unused and approach expiration.

"I would encourage every governor who has doses that they worry may be expiring to work with the FDA directly on the proper storage procedures as they continue to examine processes that will allow them to potentially last longer," White House COVID-19 Advisor Andy Slavitt said on a Tuesday press call.

Safety concerns about J&J's shot and flagging demand for vaccinations have left close to half of the 21 million doses J&J has produced for the United States sitting unused.