'We must not allow ISIS to return' to US: Trump

America must not let Islamic State jihadists return to or enter the U.S. after they are beaten overseas, President Donald Trump said Tuesday, hours after a deadly New York attack.

"We must not allow ISIS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!" Trump tweeted.

New York Mayor Bill de Blasio called the attack an "act of terror" but authorities have not definitively linked the suspect who was detained afterward to IS.

Eight people were killed and numerous others wounded when the suspect rammed a pickup truck into pedestrians in Lower Manhattan and collided with a school bus, officials said.

Trump earlier decried the attack as an act of madness.

"In NYC, looks like another attack by a very sick and deranged person," Trump tweeted.

"Law enforcement is following this closely. NOT IN THE U.S.A.!"

IS has been dealt a string of defeats across Iraq and Syria in recent months, with authorities worried that fleeing foreign fighters might return home.