ABC News' Adam Teicholz and Kari Rea report:

President Obama is "wrong" to downplay the dangers facing the United States, longtime counterterrorism official and ABC News contributor Richard Clarke said in response to the president's attempts to calm concern over the escalating threat from ISIS and turmoil in the Middle East.

Obama told an audience at a Democratic fundraiser in New York Friday that the "world has always been messy" but added, "I promise you things are much less dangerous now than they were 20 years ago, 25 years ago or 30 years ago."

Clarke, appearing this morning on "This Week," said simply, "I think he's wrong."

"We're much more capable of defending ourselves now," said Clarke, who served under both Presidents Bill Clinton and George W. Bush as the nation's top counterterrorism adviser. "We have the Department of Homeland Security, we have a lot of resources going into counterterrorism, but the threat has also increased. And I think the threat has probably increased more than the defenses."

READ: How Entrenched Is ISIS in Syria?

Speaking of the Islamic terrorist group ISIS, Clarke told ABC News' Martha Raddatz, "ISIS is highly capable. It has a lot of money. It has people from many, many countries. And our fear is it may have people in its ranks that we don't know about. We have the names of thousands of people, and we can stop them if they try to get into this country, but if we don't know their names, and we don't know they're involved, they can get in."

Other experts on "This Week" echoed such concerns. Mubin Shaikh, a former Taliban jihadist who became an undercover counterterrorism operative, said that while terrorist recruiting efforts in the United States could eventually be brought under control, "the horse has bolted from the farm" in terms of existing efforts.

Former Homeland Security Department counterterrorism coordinator John Cohen noted that recruiting efforts by groups like ISIS have become more modern, using tools such as social media and even rap music to recruit new followers.

"They are very sophisticated in their use of social media," Cohen said on "This Week." "They have Westernized their message. They're specifically seeking to either recruit or inspire Westerners, and in particular, people in the U.S… So we have to be concerned and we have to take steps to neutralize that. And it's not just countering their message, it's understanding why their message is resonating and what we can do in the community to lessen the impact that that message has."

Clarke emphasized the importance of law enforcement working with the local Muslim communities to combat those recruiting efforts, saying, "our best defense are our own American Muslims who have been very cooperative. They don't want anything to happen like this again in this country."

"It's supporting efforts at the community level to empower those communities to better recognize, better detect those individuals who may be potentially a threat and then to work together to deal with them," Cohen added.

Concerns of a potential attack became more real when Great Britain raised its terror alert level to "severe" on Friday, saying a terrorist attack from foreign fighters on the West is "highly likely."

"There is something very specific going on. They've got hundreds of individuals who have gone and become trained in Syria and Iraq with ISIS in a particularly bloody kind of terrorism, bloody kinds of attacks, who have the freedom to travel back home," said former Deputy Homeland Security Secretary Jane Holl Lute on "This Week." "So everyone is on alert."

Like "This Week" on Facebook here. You can also follow the show on Twitter here.

Go here to find out when "This Week" is on in your area.