New video shows apparent execution of 150 Syrians; Britain raises terror threat level to 'severe'

Armed police officers patrol during terror alert in London, England. (AP photo)

LONDON, England (AP/ABC NEWS) - Britain raised the terror threat level from substantial to severe Friday, meaning that a terrorist attack is considered highly likely.

Home Secretary Theresa Mays said the decision to raise the threat level was related to developments in Iraq and Syria, but that there was no information to suggest an attack was imminent. Some of the plots are likely to involve fighters who have traveled from Britain and Europe to take part in fighting in the Middle East.

May says the decision, made by the Joint Terrorism Analysis Center, is made on the basis of intelligence and is independent of government. "Severe" is the second-highest of five levels.

"We face a real and serious threat in the UK from international terrorism," May said. "I would urge the public to remain vigilant and to report any suspicious activity to the police."

The new threat level comes at the same time a new, disturbing video has surfaced from ISIS. The video appears to show captured Syrian soldiers - some 150 of them - stripped down to their underwear and forced to march across the desert after ISIS took control of the airbase they were defending.

A short while later, the video shows the soldiers appearing to lie dead, the apparent victims of a mass execution.

One of America's closest allies, the British say they support airstrikes against ISIS in Iraw, but like President Obama, Prime MInister David Cameron did not go so far as to call for airstrikes in Syria.

President Obama himself is trying to tamp down the emotional response to the ISIS threat, stressing during a briefing at the White House Thursday that developing a strategy for dealing with the terror group will take time.

"I don't want to put the cart before the horse," the President said. "We don't have a strategy yet."

It was a stark admission, since the threat of ISIS has increased at an astonishing rate in the last year, as the group has seized large areas in Iraq and Syria.

The execution of journalist James Foley prompted the President earlier this week to authorize surveillance flights over Syria.

"A group like ISIS is beyond the pale," President Obama said Thursday.

Along with the British, the President is trying to build a coalition of allies, including countries in the Middle East, to confront ISIS - and that will take come patience, he says.

In the meantime, British police have appealed to the public to help identify aspiring terrorists, particularly after it was said the person who executed Foley in the gruesome video is most likely British. That discovery underscored the need to identify those who might travel abroad to fight or are at risk of being radicalized.