Four people were injured, including one critically, when a fire broke out in an Upper East Side building early Wednesday morning, authorities said.

The fire started at 4:45 a.m. inside the six-story building on East 78th Street between First and Second avenues, officials said.

Another person suffered serious injuries, along with a firefighter and one other civilian who both had minor injuries, authorities said.

All four were rushed to New York Hospital, officials said.

The cause of the blaze is still under investigation.