President Donald Trump is promising to hit back against the Islamic State group "10 times harder" after its followers attack the U.S., like Tuesday's New York City truck massacre.

Before boarding Marine One at the White House, Trump says the effort is already ongoing: "So when we have an animal do an attack like he did the other day on the West side of Manhattan, we are hitting them 10 times harder."

Trump is taunting the Islamic State group as he promises retaliation: "They claimed him as a solider, good luck."

It is not immediately clear whether Trump's promise of retaliation refers to specific strikes. Eight people were killed this week when a man steered a rented truck down a bike bath. He has been arraigned on terrorism charges