Prime Minister David Cameron on Monday pledged a wide response to extremism, describing Britain as being united in grief over the beach massacre in Tunisia.

Fifteen Britons are confirmed dead, but officials have warned that could rise as high as 30, making it the worst terror attack on U.K. citizens since the July 7, 2005, London transport attacks that killed 52 commuters. Cameron insisted the nation would not be cowed.

"To our shock and grief we must add another word: resolve. Unshakeable resolve. We will stand up for our way of life," Cameron wrote. "So ours must be a full-spectrum response - a response at home and abroad; in the immediate aftermath and far into the future."

Some 600 British counterterrorism police — one of the force's largest such deployments in recent years — have been deployed as part of the investigation into Friday's attack at the Imperial Marhaba Hotel in the beach resort of Sousse. Officers have also been deployed at airports to meet and support travelers and to help identify witnesses.

Home Secretary Theresa May is traveling to the north African nation for talks on the extremist threat and to offer condolences for the slain tourists. A Royal Air Force transport plane is also being deployed to bring stranded families home. Cameron told the BBC that the government is working as fast as possible to give families information.

Cameron described the Islamic State group — sometimes called ISIL — as using ancient barbarism and combining it with propaganda — using social media as its primary weapon.

"We must look at how we can work with countries like Tunisia to counter this online propaganda," he wrote. "We must also deal with it at its source, in places like Syria, Iraq and Libya, from where ISIL is peddling and plotting its death cult."