British Muslims are "absolutely not reporting" suspected terrorists, Donald Trump said as he pledged to "hit Isis so hard you wouldn't believe it" if he becomes US president.
The front-runner for the Republican nomination told ITV's Good Morning Britain that Muslim communities were not doing enough to prevent attacks like the carnage in Brussels.
"When they see trouble they have to report it. They are not reporting it. They are absolutely not reporting it and that is a big problem," he told the programme's presenter Piers Morgan.
More than half a million people signed a petition calling for the tycoon-turned-politician to be banned from the UK after he demanded a block on Muslims entering the US and claimed parts of London were "so radicalised" police were "afraid for their own lives".
He said: "I would hit Isis (another name for Islamic State) so hard you wouldn't believe it and I would get the people over there to put up their soldiers because it's about time that somebody did it.
"But I would have such back-up like you've never seen before in terms of air power, air strikes etc."
"It's like they are protecting each other but they are really doing very bad damage," Mr Trump said, complaining about the failure to turn in those who went on to launch a deadly attack in San Bernardino, California.
"They have to open up to society, they have to report the bad ones. And, you know, if you report the bad ones, all of a sudden you are not going to have the problems," he said.
Mr Trump denied he was racist, telling the programme: "I'm not 'anti' anything; I'm just common sense, I say it like it is."
"I have great respect for Muslims, I have many friends that are Muslims. I am just saying there is something with a radicalised portion that is very, very bad and very dangerous."
He said he was astounded at the failure of anyone to turn in Salah Abdeslam, the chief suspect in the Paris terror attacks who was holed up in his own neighbourhood in the Belgian capital.
"There is something wrong, and we have to get to the bottom of it, when someone like who was just captured was really coddled and taken care of by people that live in the neighbourhood.
"Many people knew he was there yet he was the number one fugitive in the world. Everybody from that area knew he was there and nobody turned him in.
"There's something going on and there's something wrong. You know it, Piers, and so do I."
He went on: "I knew Brussels years ago. I was there probably three or four times and it was so beautiful, so secure and so safe. Now it's an armed camp," he said.
"It's like a different world, a different place, there is no assimilation.
"What's going on there and what's going on in many other locations - like Paris and others - is a disgrace, an absolute disgrace that we allowed to happen."
He rejected Prime Minister David Cameron's claim that his call to ban Muslims risked aiding extremists by sowing division.
"All you have to do is look at the cities where there's been a large inflow and something's different," he said.
"There is very little assimilation for whatever reason ... they want to go by their own sets of laws."
Mr Trump opposed the Iraq War but said military intervention was required against Islamic State.
"You've got to take them out harshly and you've got to take them out fast. You have no choice. We're now in the age of chopping off heads. Nobody would have thought that was possible."
Mr Trump told Mr Morgan - who said he considered the billionaire "a good friend" he had known for a decade - that he did not believe many people in the UK were scared of the idea of him in the White House.
"I don't think too many are. Honestly, I'm a very normal person. I happen to be intelligent, very intelligent, I guess, based on certain results that I get in doing things," he said.
"I am a person that just wants to see things run properly. I don't want to see people hurt."
Mr Trump's statements are "just not true", said Miqdaad Versi, assistant secretary-general of the Muslim Council of Britain.
He told Good Morning Britain: "What we have to recognise is when some of these statements are made that fuel this idea of bigotry and really fuel the thing that terrorists themselves want - that Muslims are apart from the West and cannot be seen as equal citizens - these things are not good for our society."
Mr Versi said he cannot speak for the rest of Europe but said in London it is "not the case" that Muslims are not reporting people they suspect to be extremists, adding that there is a "much more integrated society" in the UK.
He said: "In the UK British Muslims have been very outspoken on this issue."
Mr Versi said research had shown that the overwhelming majority of Muslims in Britain would report suspicious activity.
He warned it was concerning that people are confusing criminality and religion.
He said: "We have to understand how much of this is due to them being Muslim communities or just a community of criminality which people are working within? And we have to try and not conflate the two together."
Mr Versi warned that bigotry against Muslims is growing, describing it as "a serious concern".
"That's dangerous for the cohesiveness of our society."
The Metropolitan Police's Deputy Assistant Commander Neil Basu said police had received "increasing volumes of calls" to the dedicated anti-terror hotline since it was set up, which he described as a "measure of success".
He told Good Morning Britain: "We know we are getting more referrals into our Prevent programmes which are aimed to dissuade people from radicalisation.
"So I think it's working but this is a generation of work."
Asked whether an attack in the UK is inevitable he said he would not "patronise the public by saying that we are going to be capable of stopping every attack" but highlighted that police had prevented seven attacks in a period of 12 months.