Four people have been confirmed dead and a number of others are at the Tree of Life synagogue in Pennsylvania.
Three police officers responding to the incident are among those who have been shot, a police spokesperson said.
The attacker opened fire at the synagogue today during a prayer service when it was full of people, before being taken into custody.
Emergency services surrounded the synagogue following the reports of an active shooter.
And members of the public were urged to avoid the area.
Pittsburgh police Commander Jason Lando said: “Do not come out of your home right now, it is not safe.”
Authorities have not yet confirmed the number of casualties.
US President Donald Trump said on Twitter: “Watching the events unfolding in Pittsburgh, Pennsylvania. Law enforcement on the scene. People in Squirrel Hill area should remain sheltered. Looks like multiple fatalities. Beware of active shooter. God Bless All!”
Dramatic pictures show multiple police cars and ambulances at the scene.
One Twitter user said: “There was just a man who opened fire in the synagogue down the street from my job. We’ve seen 15+ police cars drive by.”
The Pittsburgh Public Safety Department told the public to avoid the area.
It said on Twitter: “There is an active shooter in the area of WILKINS and Shady. Avoid the area. More info will be released when it is available.”
Witnesses said the gunman walked into the synagogue and opened fire.
Emergency services arrived at the scene of the incident at about 10am local time (2pm UK time) on Saturday.
An emergency responder said on local television station KDKA that was simulcast on CNN: “There are fatalities.”
The conditions of the officers were not immediately known.
One white male suspect was in custody, KDKA said, adding that responding officers “received fire.”
Earlier, a police commander said the shooting resulted in “multiple casualties”.
Police surrounded the Tree of Life synagogue after reports of an active shooter at the building in the city’s Squirrel Hill neighborhood, local TV news images showed.
Pittsburgh police Commander Jason Lando warned local residents, in an impromptu news conference at the scene: “Do not come out of your home right now, it is not safe.”