U.S. President Donald Trump delivered his first public condemnation of anti-Semitic incidents in the United States on Tuesday after a new spate of bomb threats to Jewish community centres around the country and vandalism in a Jewish cemetery.

Several of the centres were evacuated for a time on Monday after receiving the threats, the JCC Association of North America said, and another centre was evacuated on Tuesday morning in San Diego, California, according to police.

Also, vandals toppled about 170 headstones at the Chesed Shel Emeth Society cemetery in St. Louis, Missouri, over the weekend.

"The anti-Semitic threats targeting our Jewish community and community centres are horrible and are painful and a very sad reminder of the work that still must be done to root out hate and prejudice and evil," Trump told reporters.

He was speaking at the end of a tour of the National Museum of African American History and Culture in Washington, which Trump said showed "why we have to fight bigotry, intolerance and hatred in all of its very ugly forms."

The comments marked a change for Trump, who had not explicitly and publicly condemned the threats against Jews when asked last week. Instead, he spoke more generally about his hopes of making the nation less "divided."

The president reacted with anger at a news conference last week when a journalist from a Jewish magazine asked how his government planned to "take care" of a rise in threats.

Trump berated the reporter for asking a "very insulting" question, appearing to believe the reporter was accusing him of being anti-Semitic.

"Number one, I am the least anti-Semitic person that you've ever seen in your entire life," the president said, adding that he was also the least racist person. Trump has often noted that one of his daughters is a convert to Judaism, he has Jewish grandchildren and he employs many Jews in his business.

Trump's daughter Ivanka, a close adviser to her father who practices Orthodox Judaism, responded to the latest threats in a message on her Twitter account on Monday evening.

"America is a nation built on the principle of religious tolerance," she said. "We must protect our houses of worship & religious centres."

On Tuesday, Trump again declined to answer a question about what action he would take to address the threats to Jewish organizations. Sean Spicer, a White House spokesman, said later that Trump would respond through "deed and action" over the coming months and years.

'BAND-AID'

Trump's derogatory campaign rhetoric against Muslims and Mexican immigrants won enthusiastic backing from prominent white supremacists who embrace anti-Jewish, anti-black and anti-Muslim ideologies. It also drew greater media attention to fringe extremist groups.

Trump has disavowed their support. His chief strategist, Steve Bannon, is the former publisher of Breitbart, a news website popular among right-wing extremist groups.

The Anne Frank Center for Mutual Respect in New York, which has criticized the Trump administration repeatedly over anti-Semitism, said his comments were too little too late.

"The president's sudden acknowledgement is a Band-Aid on the cancer of anti-Semitism that has infected his own administration," Steven Goldstein, the group's executive director, said in a statement.

Spicer rejected the characterization.

"I wish that they had praised the president for his leadership in this area," he told reporters when asked about Goldstein's comment. "Hopefully as time goes by they'll recognise his commitment to civil rights."

Jewish groups criticized the White House for omitting any mention of Jews in its statement marking Holocaust Memorial Day last month. The White House said the omission was deliberate since the Nazis also killed people who were not Jews, if in smaller numbers. The stated goal of the Nazis was the extermination of Jews.

One day after speaking at a security summit in Munich, U.S. Vice President Mike Pence spent Sunday morning walking through the grounds of the Dachau concentration camp in Germany with a camp survivor.

Over the course of the U.S. Presidents Day holiday on Monday, bomb threats were sent to 11 Jewish community centres, including ones in the Houston, Chicago and Milwaukee areas, according to the JCC association. They were found to be hoaxes, as was another threat that forced the evacuation of a centre in San Diego on Tuesday morning, according to police.

No arrests were made. The FBI has said it is investigating recent threats as "possible civil rights violations."

The Council on American-Islamic Relations, a prominent Muslim human rights group, has offered a $5,000 reward for information leading to the conviction of anyone behind the threats, saying Muslims felt a duty to support any targeted minority group.

The incidents on Monday followed three waves of bomb threats so far this year. In all, at least 69 incidents at 54 Jewish community centres in 27 states and one Canadian province have been reported, according to the JCC association.