British Prime Minister David Cameron
British Prime Minister David CameronReuters

British Prime Minister David Cameron has said that ISIS terrorists are actively planning terrorist atrocities against the UK, and warned the group poses an existential threat to the western world.

Speaking just days after a massacre by an ISIS gunman on a beach in Tunisia which killed 38 people - at least 30 of them British citizens - Cameron warned that until ISIS was defeated in Syria and Iraq, it would continue to pose a deadly threat to the UK and beyond,

"There are people in Iraq and Syria who are plotting to carry out terrible acts in Britain and elsewhere and as long as ISIL (Islamic State) exists in those two countries we are at threat," Cameron told BBC radio, using an alternative acronym for the Islamic State group.

British politicians have described Friday's attack on the Sousse seaside resort as the worst terrorist atrocity against British citizens since the 2005 Al Qaeda suicide bombings in central London.

Tunisian investigators say they know terrorist Seifeddine Rezgui did not act alone, and are currently seeking his accomplices.

The attack took place on the same day as a series of other attacks, at least one of which was also claimed by an ISIS-affiliate.

At least 27 people were killed in a suicide bombing at a Shia mosque in Kuwait on the same day. The bomber is believed to have been a Saudi national, and a group linked to Islamic State claimed the attack shortly after.

Also on Friday, a Muslim terrorist beheaded his boss and injured several others in a gruesome attack in the French city of Lyon.

ISIS has encouraged its followers to carry out attacks during the Muslim holy month of Ramadan.