US Secretary of State John Kerry has said that "no one country" can stop violent extremism.

Delivering the opening remarks on the second day of a White House summit on the prevention of violent extremism, Kerry said militants bent on attacks are a formidable foe that must be countered.

"Terror is their obsession. It's what they do. And if we let them, their singleness of purpose could actually wind up giving them a comparative advantage," Kerry said at a State Department ministerial conference.

"But with the images of recent outbreaks fresh in our minds, everybody here knows we simply can't let that happen. We have to match their commitment and we have to leave them with no advantage at all."

The three-day summit, which started on 17 February and brings together local officials from across the country and ministers from around the world, follows recent shootings in Copenhagen and Paris that have stiffened Western resolve against extremist attacks.

"No one country, no one army, no one group is going to be able to respond to this adequately. And we see that in the numbers of countries that are now being touched by it," Kerry said.

Speaking at the summit on Wednesday, President Barack Obama called on American Muslim communities to do more to counter what he called "violent extremism".

Critics have accused the White House of shying away from tying extremism to the religion of Islam following the Paris and Copenhagen attacks, both of which were perpetrated by Islamic militants.