U.S. President Donald Trump's sons joined in on the volley of criticism their father has directed at London's mayor over the attack on June 3 that killed seven people, the latest in a series of Transatlantic sniping that extends back 18 months.

In an interview with ABC's Good Morning America on Tuesday morning, Donald Trump Jr. said London's Mayor Sadiq Khan "should do something to fix the problem rather than just sit there and pretend there isn't one."

Trump's middle son, Eric Trump, said, "This has become the new norm. And it's not right. And we, as a society, especially as Americans, better do something about it."

The deadly attack began when a van ran over pedestrians on London Bridge. Police say three men wearing fake suicide vests and armed with knives jumped out and stabbed people in the Borough Market.

The president spent much of the weekend responding to the attack on Twitter. He originally levelled an inaccurate criticism of Khan, saying the mayor was telling people there was "no reason to be alarmed" about the attack.

At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!" —@realDonaldTrump

The mayor had instead been telling London residents not to be concerned by a stepped-up police presence in the city following the attack.

"No reason to be alarmed," Khan said, describing a more visible presence as "one of things the police and all of us need to do to make sure we are as safe as we possibly can be."

A spokesperson for the mayor would then say that Khan was too busy to respond to Trump's "ill-informed" tweet.

Trump continued the criticism of Khan on Monday, calling his efforts to calm the public a "pathetic excuse."

Meanwhile London's previous mayor Boris Johnson, defended Khan's original tweet on Tuesday.

Flamboyant British foreign secretary speaks after critical comments emerge 0:28

"I think it is entirely right that Sadiq, as the mayor of the greatest city on Earth, should reassure his public and his citizens about the presence of armed police on the streets," said Johnson, now Britain's foreign secretary.

"He was making a very sensible point and he's entirely right to do so."

Those comments echoed those of British Prime Minister Theresa May, who told the Sun newspaper, "I think Donald Trump is wrong in what he said about Sadiq Khan, in relation to the attack on London Bridge."

'Total and complete shutdown'

The roots of the back-and-forth sniping date back to December 2015 and an attack in San Bernardino, Calif., in which 14 people were killed by a gun-wielding Muslim husband and wife.

Trump on the campaign trail ahead of the Republican primaries called for a "total and complete shutdown of Muslims entering the United States." Those comments have continued to resonate, affecting his government's legal arguments for a proposed ban on travel to the U.S. for citizens of six countries.

Khan, British born to parents from Pakistan called Trump a "buffoon" at the time, but said he would like to educate him in a debate or meeting about Islam.

"There are many Muslims from around the world who love America, like I do, but who'll be stopped from going there to visit their families, for their work or on holiday," said Khan, then a candidate for London mayor.

"I think his views are divisive, they are outrageous and I hope he loses badly," Khan added, emphasizing the last word.

No exception, no rule

Khan would go on to win the mayoral race in May 2016.

Trump congratulated Khan on his victory, stating he was "happy to see that." He said Khan could "lead by example" for Muslims and, not to worry, he would be one of the "exceptions" to the Muslim ban.

Khan rejected being singled out, arguing it was possible to be both a mainstream Muslim and live in Western society.

Donald Trump Jr., right, has previously criticized Sadiq Khan for his alleged softness on terrorism. On Tuesday, both he and brother Eric Trump, left, continued the criticism in a televised appearance. (Kathy Willens/Associated Press)

"It's not just about me," he said. "Don't make an exception for me, reconsider your views on Islam."

In an interview days later with ITV, Trump said Khan's comments were "very nasty."

"Tell him I will remember those statements," Trump said.

The other Chelsea

On Sept. 18, 2016, a bomb went off in the Chelsea neighbourhood of New York City. It was one of multiple explosions in New York and nearby New Jersey, leaving a few dozen injured.

When asked in England about the attacks, Khan gave his thoughts and prayers to the citizens of New York and then gave the comments that would give the Trumps, and Fox News, fodder well into future.

"What I do know — it's part and parcel of living in a great global city; you've got to be prepared for these things. You've got to be vigilant. You've got to support the police who do an incredibly hard job [and] you've got to support the security services."

You have to be kidding me?!: Terror attacks are part of living in big city, says London Mayor Sadiq Khan <a href="https://t.co/uSm2pwRTjO">https://t.co/uSm2pwRTjO</a> —@DonaldJTrumpJr

When a lone attacker killed five people in London on the Westminster Bridge and its surroundings in March, Donald Trump Jr. tweeted a link to those comments by Khan six months earlier.

Critics pounced on the president's son for leaving out the "vigilant" part of Khan's quote and giving the impression the London mayor was resignedly giving in to terrorist attacks without trying to prevent them.

Reporters at the White House on Monday questioned the optics of lashing out at an official in another country who was busy responding to a crisis and grieving for citizens and tourists killed in his constituency.

Trump visit to U.K. not yet set

White House spokesperson Sarah Huckabee Sanders at first denied that Trump misinterpreted Khan's "no reason to be alarmed" statement.

"I don't think that's actually true, I think that the media wants to spin it that way," said Sanders.

In response to a specific question, she said the suggestion Khan has been in the crosshairs of the Trumps because he is a Muslim is "utterly ridiculous."

Hillary Clinton, defeated by Trump in November, also questioned why the U.S. president would seek to damage a relationship with an ally, according to the Baltimore Sun.

"This is not a time to lash out, to incite fear, or to use tragedy and terror for political gain," the newspaper reported Clinton as saying while making an appearance at a charity event in Baltimore on Monday.

The comments by the Trump brothers aren't likely to be the last in the feud.

It has been widely speculated that Trump will make a state visit to Britain in the fall. Khan on Monday said that invitation should be reconsidered.

"I don't think we should roll out the red carpet to the president of the U.S.A. in the circumstances where his policies go against everything we stand for," he said.

Tim Farron, leader of the Liberal Democrats in the U.K., has labelled Trump "an embarrassment to America."

He posted a message on Twitter stating: "Theresa May must withdraw the state visit. This is a man insulting our national values at a time of introspection and mourning."

Both May and Johnson have said they have no plans on scrapping the visit, given the importance of the relationship between the U.S. and the U.K.