A gunman yelling "all Jews must die" has stormed a Pittsburgh synagogue during Saturday sabbath services and shot worshippers, killing at least 11 people and injuring six more.

Key points: The attack took place during a baby naming ceremony

The attack took place during a baby naming ceremony Suspect named as Robert Bowers traded gunfire with police and was shot several times

Suspect named as Robert Bowers traded gunfire with police and was shot several times Donald Trump says "some kind of a protection inside the temple" may have reduced casualties

A suspect reported to be a "bearded heavy-set white male" was taken into custody after the shooting at the Tree of Life synagogue in the Squirrel Hill neighbourhood in the US state of Pennsylvania.

The suspect, Robert Bowers, traded gunfire with police, injuring four officers, and he was shot several times in the 20-minute attack.

Mr Bowers, aged in his 40s, is in hospital in a fair condition.

FBI agent Bob Jones told a press conference Mr Bowers used an assault rifle and three handguns in the attack, but there was no indication he was known to law enforcement before the shooting.

Mr Jones, who described the shooting as "the most horrific crime scene" he had seen in 22 years with the FBI, said investigators believed Mr Bowers was acting alone.

He said the full motive remained unknown.

Sorry, this video has expired FBI agent Bob Jones described the shooting as the most horrific crime scene he had seen in 22 years.

US Attorney-General Jeff Sessions said the Justice Department would file hate crime charges against Mr Bowers that "could lead to death penalty".

SWAT police officers respond after a gunman opened fire at the Tree of Life synagogue. ( Reuters: John Altdorfer )

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The attack took place during a baby naming ceremony.

"Do not come out of your home right now, it is not safe," Pittsburgh police commander Jason Lando warned local residents.

Michael Eisenberg, former president of the synagogue, told KDKA that police were normally only present at the synagogue for security on Jewish holidays.

"On a day like today, the door is open, it's a religious service, you can walk in and out," he said.

The shooting came amid a rash of high-profile attacks in an increasingly divided country, including the series of pipe bombs mailed over the past week to prominent Democrats and former officials.

The FBI says the motive for the attack remains unknown. ( AP: Stephanie Strasburg/Pittsburgh Post-Gazette )

Threat made against Jewish refugee organisation

The social media site Gab.com said Mr Bowers had a profile on its website, which is popular with far-right extremists.

The company said the account was verified after the shooting and matched the name of the gunman.

A man with the same name posted on Gab before the shooting that: "HIAS likes to bring invaders in that kill our people. I can't sit by and watch my people get slaughtered. Screw your optics, I'm going in."

A screenshot of a post from a user on the social media website Gab by a user named Robert Bowers. ( AP )

HIAS is a non-profit group that helps refugees around the world find safety and freedom. The organisation says it is guided by Jewish values and history.

Mr Bowers also recently posted a photo of a collection of three semi-automatic handguns he titled "my Glock family", a reference to the firearms manufacturer.

He also posted photos of bullet holes in person-sized targets at a firing range, touting the "amazing trigger" on a handgun he was offering for sale.

Anti-Defamation League chief executive Jonathan Greenblatt called the shooting an "anti-Semitic attack" and said the group believed it was the deadliest assault on the Jewish community in US history.

"Our hearts break for the families of those killed and injured at the Tree of Life Synagogue, and for the entire Jewish community of Pittsburgh," Mr Greenblatt said.

People hold candles as they gather for a vigil outside the Tree of Life synagogue in Pittsburgh. ( AP: Matt Rourke )

Trump says synagogue should have had 'protection'

US President Donald Trump told reporters the shooting had little to do with gun laws.

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"If they had some kind of a protection inside the temple maybe it could have been a much more different situation. They didn't," he said when asked about a possible link to gun laws.

Mr Trump told reporters the shooting showed that the US should stiffen laws on the death penalty.

"When people do this they should get the death penalty," he said.

He also used the terms "madman" and "wacko" when talking about the shooting.

Police run with a person on a stretcher outside the Tree of Life synagogue in Pittsburgh. ( AP: Alexndra Wimley/Pittsburgh Post-Gazette )

Pennsylvania Governor Tom Wolf described the shooting as "an absolute tragedy".

"These senseless acts of violence are not who we are as Americans," he said in a tweet.

"We must all pray and hope for no more loss of life. But we have been saying 'this one is too many' for far too long. Dangerous weapons are putting our citizens in harm's way.

"We cannot accept this violence as normal."

Mr Trump later spoke at a rally in south Illinois for Republican Mike Bost, saying cancelling his appearance would make "sick, demented people important" and calling the shooting an "evil anti-Semitic attack".

Speaking to a massive, cheering crowd at an airport hangar, Mr Trump said "the hearts of all Americans are filled with grief, following the monstrous killing".

Israeli Prime Minister Benjamin Netanyahu said he was "heartbroken and appalled" by the shooting.

"We stand together with the American people in the face of this horrendous anti-Semitic brutality and we all pray for the speedy recovery of the wounded," he said in a statement.

World Jewish Congress president Ronald S Lauder called the shooting "an attack not just on the Jewish community, but on America as a whole".

A building in Tel Aviv is lit in the colours of the American flag in solidarity with the victims of the Pittsburgh attack. ( Reuters: Nir Elias )

Synagogues had trained for active shooters

Mr Eisenberg was getting ready for services when he received a phone call from a member who works with Pittsburgh's Emergency Services, saying he had been notified through scanner and other communications that there was an active shooter at their synagogue.

"I ran out of the house without changing and I saw the street blocked with police cars. It was a surreal scene. And someone yelled, 'Get out of here.' I realised it was a police officer along the side of the house … I am sure I know all of the people, all of the fatalities. I am just waiting to see," Mr Eisenberg said.

A baby naming ceremony was taking place in the synagogue when the gunman opened fire. ( AP: Gene J. Puskar )

He said officials at the synagogue had not gotten any threats that he knew of prior to the shooting.

The synagogue maintenance employees had recently checked all of the emergency exits and doors to make sure they were cleared and working.

"I spoke to a maintenance person who was in the building and heard the shots. He was able to escape through one of the side exit doors we had made sure was functioning," Mr Eisenberg said.

Jeff Finkelstein of the Jewish Federation of Greater Pittsburgh said local synagogues had done "lots of training on things like active shooters, and we've looked at hardening facilities as much as possible".

"This should not be happening, period," he told reporters. "This should not be happening in a synagogue."

Four responding police officers were shot during the attack. ( Reuters: John Altdorfer )

Reuters/AP