"I think there are a lot of problems in Europe that are very, very severe," he said
Donald Trump has claimed Britain and Europe are "not safe places" following recent terrorist attacks.
The front runner for the Republican presidential nomination made the comments five days after the Brussels atrocities, which killed at least 31 people.
The real estate tycoon said Europe had lots of "very, very severe" problems and added he did not even think America was a safe place for Americans.
Mr Trump told ABC: "I don't think Bruss - England or I don't think that Europe is a safe place. No, I don't.
"I think there are a lot of problems in Europe that are very, very severe.
"When you look at Brussels, when you look at the way they've handled things from law enforcement standpoints, when you look at Paris, when you look at so many other places, no, it's not (safe)".
John Kerry, the US Secretary of State, echoed the comments, telling CBS's Face The Nation US citizens should "avoid a crowded place" if they were travelling in Europe, because "you have no control over who may be there".
Mr Trump was mocked last year for his comments on "no-go" areas in London, when he claimed some areas of the city were "so radicalised that the police are afraid for their own lives".
He also caused controversy earlier this week, when he said British Muslims are not reporting suspicious activity by extremists to the authorities.
The billionaire businessman's latest remarks came as Belgian officials completed their initial investigation at Brussels International Airport and found the main building to be stable.
Airport officials are now discussing plans to install temporary check-in desks, but in a statement the airport said that the process of identifying a suitable location was still being carried out in consultation with airlines and luggage handlers.
Earlier on Sunday, a man who was shot by police was charged with membership of a terror group.
Belgian prosecutors named him as A Abderrahmane, who was shot in the leg when he failed to respond to police demands to raise his arms in the Brussels district of Schaerbeek on Friday.
He is thought to have been involved in a planned attack on France that was foiled following a raid last week.
Meanwhile, riot police were forced to used water cannons on a far-right protest group which disrupted a large remembrance gathering at the central Place de la Bourse in the Belgian capital.