Image copyright Reuters

Foreign Secretary Boris Johnson has urged allies fighting the so-called Islamic State group to do more to gather evidence of war crimes.

Speaking at a conference in Washington on his first US visit in his new role, Mr Johnson said the UK would lead a campaign to bring the group to justice.

He warned of the potential dispersal of IS fighters around the world after they are pushed out of Iraq and Syria.

Mr Johnson proposed a UK summit to examine how to tackle the new threats.

Addressing foreign and defence officials from about 30 nations involved in the fight against IS, Mr Johnson said more needed to be done to collect evidence in territory the group has lost.

Witnesses would need to be identified and data collected so individuals could be held to account and prosecuted, he said.

Held to account

Later, in a TV interview, Mr Johnson said: "We've got to deal with the whole cancer and its ability to spread and to metastasize, to pop up all over the world in the way that we've been seeing...

"There are thousands of them and we need to start setting in train the process of gathering evidence, of getting more witnesses, so that ultimately they can be prosecuted and held to account for their crimes against humanity and that's something that I said today to everybody and got a large measure of support."

Image copyright AFP Image caption US Secretary of State John Kerry hosted the gathering of the anti-IS coalition in Washington

Mr Johnson also defended Nato's principle of mutual self-defence following a suggestion by Republican presidential nominee Donald Trump that the US might not always come to the aid of the group's fellow members.

Outlining a foreign policy strategy aimed at reducing US expenditure and involvement abroad, Mr Trump said as president he may abandon a guarantee of protection to fellow Nato members unless they have "fulfilled their obligations to us".

Mr Johnson said Nato's "doctrine of mutual defence is incredibly important. It is something I have repeated several times in the last week to people around Europe, to representatives and my counterparts, the Baltic countries and elsewhere.

"It is something that the British government believes in absolutely fervently and something we stand behind four square."