A gunman opened fire on Jewish worshippers at a synagogue in the US city of Pittsburgh yesterday, leaving 11 people dead according to US media, in what appeared to be one of the worst anti-Semitic attacks in American history.

The shooter -- identified by media as a 46-year-old white man -- reportedly yelled anti-Jewish slurs as he launched the attack at the Tree of Life synagogue during a baby naming ceremony.

The suspect was taken into police custody and transferred to hospital.

Dozens of people were gathered during Sabbath services at the time of the attack, which federal investigators were treating as a hate crime.

President Donald Trump hit out at a climate of "hate" in America and beyond -- while his daughter Ivanka, a convert to Judaism, denounced a "depraved" attack and Vice President Mike Pence called it "an assault on our freedom of religion."

"There must be no tolerance for anti-Semitism," the president told supporters at a campaign event for next month's midterm elections. "This wicked act of mass murder is pure evil, hard to believe, and, frankly, something that is unimaginable."

"Our nation and the world are shocked and stunned by the grief."

As police cars, ambulances and a SWAT team surrounded the synagogue in the historically Jewish neighborhood of Squirrel Hill, Wendell Hissrich, Pittsburgh's public safety director, confirmed "multiple fatalities" and at least six people injured including four police officers whose condition was not immediately clear.

The Washington Post and the local CBS affiliate said 11 people were dead.

"The scene is very bad inside," Hissrich said. "It's a very horrific crime scene, one of the worst that I've seen, I've been on plane crashes. It's very bad."

A woman on site told CNN her daughter was with others who ran down the stairs and barricaded themselves in the basement of the synagogue after hearing shots.

"They're safe, but they kept hearing them firing and everything else," she told the television network.

Multiple US media identified the shooter as Pittsburgh resident Rob Bowers, whose online posts were reportedly rife with anti-Semitic comments.

OUR HEARTS BREAK

The attack comes at a time of sharp tensions in the United States -- a day after a Trump fan from Florida was arrested for mailing explosive devices to a series of Democratic figures, setting the country on edge ahead of close-fought elections on November 6.

Trump -- who has been accused of fueling tensions with his divisive rhetoric -- briefly considered scrapping a campaign rally later Saturday over the Pittsburgh attack, but finally decided to maintain it.

"We can't let evil change our lives and change our schedule," he said. "You go with a heavy heart but you go."

Israeli Prime Minister Benjamin Netanyahu voiced his grief and solidarity with the United States, saying he was "heartbroken and appalled."

"The entire people of Israel grieve with the families of the dead" Netanyahu said in a video message. "We stand together with the Jewish community of Pittsburgh. We stand together with the American people in the face of this horrendous anti-Semitic brutality."

"We are devastated," tweeted Jonathan Greenblatt, the head of the Anti-Defamation League, an American civil rights group, which said it was "likely the deadliest attack on the Jewish community in the history of the United States" and certainly the deadliest since 2014.

"Jews targeted on Shabbat morning at synagogue, a holy place of worship, is unconscionable. Our hearts break for the victims, their families, and the entire Jewish community," Greenblatt said.

The Pittsburgh attack comes with the United States witnessing a sharp spike in anti-Semitic incidents, having surged 57 percent from 2016 to 2017, to 1,986 from 1,267, according to the ADL.

Trump's daughter Ivanka, in a tweet, vowed that "America is stronger than the acts of a depraved bigot and anti-Semite."

"All good Americans stand with the Jewish people to oppose acts of terror & share the horror, disgust & outrage over the massacre in Pittsburgh."

It is the latest shooting incident in the United States, where gunmen regularly cause mass casualties and firearms are linked to more than 30,000 deaths annually.

The Tree of Life Synagogue, whose congregation was founded more than 150 years ago, is in a residential neighborhood about five miles (eight kilometers) east of downtown Pittsburgh.

Michael Eisenberg, past president of the synagogue, told local television the door would typically have been open on Saturdays with religious services going on. He said police are normally deployed only on High Holy Days -- the holiest annual Jewish religious holidays.

He said security was a "major concern" during his stint as president, and active shooting situations and trainings were conducted, "if something horrific like this happened."

Squirrel Hill has historically been the center of Jewish life in the greater Pittsburgh and is home to 26 percent of all Pittsburgh-area Jewish households, according to a study from Brandeis University.

More than 80 percent of neighborhood residents said they had some concern or were very concerned about rising anti-Semitism, found the 2017 study.