(CNN) Once again, President Donald Trump is having a tough time calling out far right-wing white nationalism.

His response to the carnage in New Zealand, where 49 people died in an attack on two mosques, is also raising fresh questions about his attitude toward Islam following a long history of anti-Muslim rhetoric -- and about the extent to which the President has a responsibility to moderate his language given the rise in white supremacy movements across the world.

On Twitter and in remarks in the Oval Office, Trump was clear in condemning the killings. But he did not deliver a message of empathy and support to American Muslims, who may feel scared as security is stepped up at US mosques.

"I spoke with Prime Minister Ardern of New Zealand to express the sorrow of our entire nation following the monstrous terror attacks at two mosques," Trump said in the Oval Office on Friday afternoon after first condemning the attack as "a horrible massacre in the Mosques" on Twitter.

"These sacred places of worship were turned into scenes of evil killing," the President said. "We've all seen what went on. It's a horrible, horrible thing."

But asked whether he saw a worrying rise in white supremacy movements around the world, Trump said he did not, blaming a small group of people "with very, very serious problems." He also told reporters that he had not seen the manifesto linked to by a social media account that's believed to belong to one of the attackers, which mentioned Trump by name and saw him as a symbol of renewed white identity.

While the President did not reach out to Muslims around the world, his daughter offered the kind of language that might have been expected from a more conventional commander in chief.

"We join New Zealand and Muslim communities around the world in condemnation of this evil as we pray for the families of each victim and grieve together, " Ivanka Trump tweeted on Friday morning.

New Zealand Prime Minister Jacinda Ardern was asked in a news conference early Saturday what she had told Trump in their telephone call.

"He asked what offer of support the United States could provide. My message was sympathy and love for all Muslim communities," Ardern said.

White House spokeswoman Sarah Sanders called the Christchurch killings a "vicious attack of hate," though she did not specifically mention that the attack was against Muslims.

Trump's failure to do more to point out that the worshipers who died in Christchurch were Muslim represents a double standard, given that he has been much clearer in ascribing a religious motivation to other killings.

was to blame for a rise in hate crimes. Last year, after an attack on a Jewish temple in Pittsburgh, Trump spoke of an "anti-Semitic" motive in the attack, which itself sparked a debate over whether his inflammatory rhetoricwas to blame for a rise in hate crimes.

Trump has often been quick to wade in when a Muslim extremist has been a perpetrator of an attack and Muslims are not the victims, or to use such attacks to further his political arguments.

"Incompetent Hillary, despite the horrible attack in Brussels today, wants borders to be weak and open-and let the Muslims flow in. No way!" Trump, for instance, tweeted in March 2016.

And when he was running for office, he excoriated Democrats as dishonest about the motivation of Muslim extremists who conducted terror attacks.

"These are radical Islamic terrorists, and she won't even mention the word, and nor will President Obama," Trump said at a presidential debate, referring to Hillary Clinton. "Now, to solve a problem, you have to be able to state what the problem is, or at least say the name."

Equivocation on white nationalism

Trump has many times been accused of using rhetoric that emboldens extremists and dehumanizes his targets. He has used vulgar language to criticize NFL stars who took a knee during the National Anthem. In announcing his campaign, he said Mexico was sending "rapists" across the border into the US. On Friday, at the same event in which he bemoaned the attack in New Zealand, he warned of "invasions" of undocumented migrants coming across the southern border.

And Friday was not the first time that Trump has sought to downplay the threat of white nationalism.

Trump's moral leadership also came into question when he initially equivocated after he was endorsed by white supremacist David Duke during the 2016 campaign.

The President's comment Friday that white nationalism is not a growing problem contrasted with the vehemence with which other world leaders reacted, and their clear condemnations of white supremacist rhetoric and ideology.

Britain's Prime Minister Theresa May said there was no place in society for "the vile ideology that drives and incites hatred and fear."

Australia's Prime Minister Scott Morrison condemned a "violent, extremist, right-wing terrorist attack."

Ardern said the alleged perpetrator of the attack had "extremist views that have absolutely no place in New Zealand and in fact, have no place in the world."

In a tweet that posted before Trump's comments in the Oval Office, Democratic former Vice President Joe Biden -- a possible White House candidate in 2020 -- appeared to have Trump on his mind.

"Whether it is antisemitism in Pittsburgh, racism in Charlottesville, or the xenophobia and Islamophobia today in Christchurch, violent hate is on the march at home and abroad. We cannot stand by as mosques are turned into murder scenes," Biden tweeted.

"Silence is complicity," he added. "Our children are listening. The time to speak out is now."

Democratic Rep. Joaquin Castro of Texas condemned Trump for what he styled as extremist rhetoric.

"There is a cost to that. And the cost is part of what we saw today. There are people out there who are unstable that will be inspired by that and take action," Castro told Wolf Blitzer on "The Situation Room."

White House director of strategic communications Mercedes Schlapp told reporters Friday that it was "outrageous to even make that connection between this deranged individual that committed this evil crime to the President, who has repeatedly condemned bigotry, racism."

Trump's dismissal of the idea that white nationalism is on the rise contradicted warnings of his own government, and it was a characteristic example of how he ignores statistics that do not suit his political arguments.

In a May 2017 intelligence bulletin obtained by Foreign Policy magazine, the FBI and the Department of Homeland Security warned of "lethal violence" from white supremacist extremist groups.

Trump's view also does not take into account the rise of white nationalist groups in politics in Europe, which has seen large marches in some cities.