A senior British police officer has claimed up to five Britons a week are travelling to fight with Islamic State forces.

The stark warning by Metropolitan Police Commissioner Sir Bernard Hogan-Howe that returning fighters posed a terrorist threat, came as it emerged a third man from Portsmouth had been killed in Syria.

Manunur Roshid, 24, was one of five friends who travelled to the war-torn country in October last year.

Around 500 would-be British jihadists are already thought to have joined IS, which has shocked the world with its brutality, including the beheading of two British hostages.

And the revelation by Sir Bernard indicates their number continues to swell.

He said in a speech: "The advance of IS across Iraq and Syria, which happened incredibly quickly, indeed now towards Turkey, are not just the horrors of distant lands.

"We know that over 500 British nationals travelled to join the conflict. Many have returned and many will wish to do so in the coming months and perhaps in future years.

"We still have an average of five people joining them a week. Five a week doesn't sound much but when you realise there are 50 weeks in a year, 250 more would be 50% more than we think have gone already.

"Those numbers are a minimum. Those are the ones that we believe have gone.

"There may be many more who set out to travel to another country and meandered over to Syria and Iraq in a way that is not always possible to spot when you have failed states and leaky borders."

And Sir Bernard said the return of "potentially militarised individuals" to the streets of the UK "is a risk to our communities".

He backed plans revealed by the Government last month to give authorities the ability to seize the passports of those they suspect of travelling abroad to fight with terror groups at the border.

He added: "The drumbeat of terrorism in the UK has changed. It's faster and it's more intense."

Police activity has escalated to combat the terrorist threat, with "disruption" activities at ports such as stopping suspects and seizing cash up by more than 50%.

The Metropolitan Police have made 218 arrests for terrorist-related activity so far this year - an increase of around 70% compared to three years ago, Sir Bernard said.

Police and partner agencies are disrupting several terrorist plots each year, including at least one "major" conspiracy every year since 2001, Sir Bernard said.

Earlier this month, London Mayor Boris Johnson revealed that thousands of terror suspects are being spied on every day by UK security services.