With both the number of deaths and details of the synagogue's security still to be disclosed, Trump said gun control "has little to do with it" but "if they had protection inside, the results would have been far better."
Trump responded before boarding Air Force One for a speech in Indiana and commented upon his arrival.
"A lot of people killed," Trump said. "A lot of people very badly wounded." He said the attack "looks definitely like it's an anti-Semitic crime."
Richard Cohen, president of the Southern Poverty Law Center, is deploring "another horrific act of hate at a house of worship."
He says the Saturday morning shooting is reminiscent of "the slaughter of nine African American worshippers at Charleston's Mother Emmanuel Church in 2015, the killings of six Sikh worshippers at a temple in Oak Creek, Wisconsin, in 2014, and, of course, the bombing of Birmingham's 16th Street Baptist Church in 1963 that left four young African American girls dead."
Anti-Defamation League CEO Jonathan Greenblatt has tweeted: "We are devastated. 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."
Israeli Prime Minister Benjamin Netanyahu has condemned the attack on Pittsburgh's Tree of Life synagogue.
"I was heartbroken and appalled by the murderous attack on a Pittsburgh synagogue today," Netanyahu said in a video message posted on Twitter shortly after the attack, which has killed at least two people and injured six.
Netanyahu says all of Israel is grieving with the families of the dead.
Other public figures took to social media to share their condolences:
The Associated Press contributed to this report.