At least 11 people were killed and six others were injured after a gunman opened fire Saturday morning inside a synagogue in Pittsburgh, U.S. state of Pennsylvania, a city official said.

No children were among the fatalities and the six injured include four police officers but do not include the suspect, Wendell Hissrich, Pittsburgh's public safety director, told a press conference.

The initial call to 911 was made around 9:54 a.m. local time and officers were dispatched to the scene within a minute, Hissrich said.

The shooting occurred at the Tree of Life Congregation in Pittsburgh's Squirrel Hill neighborhood.

Police said the gunman, a heavy-set white male with a beard, surrendered and is in custody. The shooting occurred during a baby-naming ceremony at the synagogue, according to reports.

Attorney for Pennsylvania's Western District Scott Brady confirmed the suspect is Robert Bowers, reportedly a 46-year-old man from Pittsburgh.

Bowers' social media posts were reportedly rife with anti-Semitic content.

Brady said Bowers' actions "represent the worst of humanity" and criminal charges against him could be filed as early as Saturday.

Police sources told local CBS affiliate KDKA that the gunman opened fire while yelling "all Jews must die."

The suspect was armed with what appeared to be an assault rifle and at least three handguns.

Investigators believe the gunman was acting alone but his full motive still isn't known, said Bob Jones, the special agent in charge of the FBI's office in Pittsburgh.

Jones said the scene of Saturday's shooting at the synagogue was "the most horrific crime scene" he has seen" in 22 years with the FBI.

The shooting will be prosecuted as a hate crime and the FBI will be leading the investigation, Hissrich said earlier in the day.

The synagogue is located at the intersection of Wilkins and Shady avenues. The neighborhood of Squirrel Hill, about 10 minutes of drive from downtown Pittsburgh, is the hub of the city's Jewish community.

The synagogue offers members a "traditional Conservative service with a modern sense of family" and shabbat service on 9:45 a.m. Saturdays, according to the Tree of Life's website.

Between 50 and 60 worshippers would be there on an average Saturday morning, according to a spokesman for the Jewish community in Pittsburgh.

U.S. President Donald Trump told a rally in Indianapolis, state of Indiana, on Saturday that "there must be no tolerance for anti-Semitism."

"This was an anti-Semitic act," Trump said. "This wicked act of mass murder is pure evil, hard to believe, and, frankly, something that is unimaginable."

UN Secretary-General Antonio Guterres on Saturday strongly condemned the shooting at the synagogue in Pittsburgh, said his spokesman.

Guterres was shocked at the bloody shooting that killed 11 people. He expressed his deepest condolences to the families of the victims, said Stephane Dujarric, the spokesman, in a statement.

"The shooting in Pittsburgh is a painful reminder of continuing anti-Semitism. Jews across the world continue to be attacked for no other reason than their identity. Anti-Semitism is a menace to democratic values and peace, and should have no place in the 21st century," said the statement.