Terrorist ideology is "evolving not disappearing", British Prime Minister Theresa May said at the Group of Seven (G7) Summit of industrialised nations.

Addressing a press conference on Friday shortly after the leaders signed a joint declaration on combating terrorism and extremist violence, May said: "We agreed the threat from Daesh (or Islamic State militia) is evolving rather than disappearing."

"Make no mistake: the fight is moving from the battlefield to the internet," Xinhua news agency quoted her as saying.

The British leader said "corporations have a social responsibility to remove harmful content from the internet" which spreads "hateful" propaganda that "warps young minds".

She also said it is "vital" that Britain cooperate with its partners on counter-terrorism investigations and intelligence sharing.

May called for the pacification of Libya and Syria, where ongoing civil wars are destabilizing the Middle East and surrounding regions.

On the trade front, May said: "I reiterate the UK's abiding commitment to free trade and open markets", calling for the World Trade Organization (WTO) to "create a level playing field" to promote a fair business environment. 

Fair play is one of the ways G7 countries can "ensure all our citizens participate in the benefits of globalisation".

She went on to say that Britain remains committed to the Paris Agreement to curb climate change.

"As the G7, we stand united today in upholding the values we share to create a safe and prosperous future for all our citizens," May said.