A gunman opened fire on Jewish worshippers at a synagogue in the US city of Pittsburgh on Saturday, reportedly killing at least four people and injuring half a dozen others before being taken into custody.

Local media reported the shooter yelled anti-Jewish slurs during the attack at the Tree of Life synagogue, where dozens were understood to be gathered during Sabbath services, and which comes with the United States witnessing a sharp spike in anti-Semitic incidents.

As reports indicated as many as eight could be dead, President Donald Trump hit out at what he called a climate of “hate” — while his daughter Ivanka, a convert to judaism, denounced a “depraved” attack.

“It's a terrible, terrible thing what's going on with hate in our country frankly, and all over the world,” Trump told reporters.

“Something has to be done,” he continued, saying the perpetrators of such attacks “should get the death penalty”.

Multiple US media identified the shooter as a 46-year-old man from Pittsburgh, Rob Bowers, whose online posts were reportedly rife with anti-Semitic comments.

Wendell Hissrich, Pittsburgh's public safety director, confirmed there were “multiple fatalities” at the scene and at least six people injured including four police officers whose condition was not immediately clear. “The scene is very bad inside,” Hissrich said.

US television networks said at least four people had died, while the local CBS affiliate reported that eight people were dead.

A woman at the scene, in the historically Jewish Pittsburgneighbourhoodod of Squirrel Hill, 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. Police cordoned off the building, which was surrounded by police cars, ambulances and a police SWAT team.

'Our hearts break'

Israeli Prime Minister Benjamin Netanyahu voiced his grief and solidarity with the US, saying he was “heartbroken and appalled by the murderous attack”.

“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 antisemitic brutality.”

“We are devastated,” tweeted Jonathan Greenblatt, the head of the Anti-Defamation League.

“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.”

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. We must unite against hatred & evil. God bless those affected.”

The New York Police Department said it was deploying heavy weapons teams to houses of worship across the city in response to the Pittsburgh attack.

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 is in a residential neighbourhood about 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 active shooter trainings were conducted, "if something horrific like this happened".

Anti-Semitism and hate crimes have been on a rise in the US in recent years — with anti-Semitic incidents surging 57 per cent from 2016 to 2017, to 1,986 from 1,267, according to the ADL.

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

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

The Tree of Life congregation was founded more than 150 years ago and in 2010 merged with the five-year-old Or L'Simcha congregation.