WASHINGTON (AFP) - US Vice-President Joe Biden opened a high-profile meeting on global extremism Tuesday by facing down critics who say the White House should focus on defeating it militarily.

The summit hosted by President Barack Obama has been in the pipeline for months but has been given deeper significance after several similarly inspired attacks, including on a cultural centre and on a synagogue in Copenhagen which left two people dead at the weekend.

"We need answers that go beyond the military, we need answers that go beyond force," Biden told a group of religious, community and law enforcement leaders from around the world.

"All of us, including the United States, have to work this from the ground up," Biden said, stressing the need to "engage our communities and engage those who might be susceptible to being radicalised."

The White House has been criticised for not specifically focusing the three-day meeting on combating Islamic extremism or broader military efforts to tackle militant groups such as Islamic State in Iraq and Syria.

On Sunday, a video emerged apparently showing Islamic State militants beheading 21 Egyptian Christians in Libya.

That and the Denmark deaths came just weeks after the Paris attacks by Islamist gunmen on the Charlie Hebdo satirical weekly magazine and a kosher supermarket that left 17 people dead.

Obama's Republican political foes say his avoidance of terms like "Islamic extremism" in the wake of these attacks shows he does not understand the threat.

His administration stresses they have little to do with Islam and "absolutely no justification" in any religion.

"We are very, very clear that we do not believe that they are representing Islam," said one senior US official. "So you can call them what you want. We're calling them terrorists."

'WE HAVE EXPERIENCE'

Obama is expected to speak on Wednesday and Thursday and representatives from almost 70 countries are expected to attend.

The White House summit is expected to look at ways to combat those "inspiring, radicalising, financing or recruiting individuals or groups" to commit violent extremism.

Information sharing, countering "violent extremist narratives" via social media and judging the effectiveness of these strategies will all be discussed.

Tuesday and Wednesday's proceedings focus on domestic US efforts to counter extremism.

"The focus of today's events are making sure that violent extremism never finds a home in the communities of the United States," Biden said opening the meeting.

"I'm not suggesting to the press or any of our guests that I think America has all the answers, we just have a lot more experience.

"We are a nation of immigrants, that's who we are."

He praised the Muslim mayor of Rotterdam, Ahmed Aboutaleb, who was present and has been highly critical of those carrying out attacks in Europe.

The meeting is expected to highlight existing anti-extremist programmes in Boston, Minneapolis-Saint Paul and greater Los Angeles which involved community policing and other tactics.

On Thursday, international efforts will take centre stage, with Secretary of State John Kerry kicking off proceedings.

Britain's home secretary, the United Arab Emirates minister of states and foreign affairs and a religious scholar from Syria are among others due to take part.