The mayor of London on Thursday dismissed a tweet from President Trump's son criticizing him for saying the threat of terrorist attacks are part of living in a major city.

"I'm not going to respond to a tweet from Donald Trump Jr.," said Mayor Sadiq Khan to CNN. "I've been doing far more important things over the last 24 hours."

Trump Jr. criticized Khan in the wake of a terrorist attack that killed four people and the attacker for saying in 2016 that the threat terrorist attacks are "part and parcel" of living in a big city and that thinking about those attacks keep him up at night. Khan was speaking about a bombing in New York City and warned Londoners to be vigilant.

In a tweet, Trump Jr. simply wrote, "You've got to be kidding me?!" with a quote of Khan's words.

Trump Jr. is in charge of the Trump Organization as his father serves as president. He remains an outspoken supporter of his father on Twitter and on TV.

One American died in the attack on London on Wednesday, the one-year anniversary of bombings in Brussels that killed four Americans.