British tourists are feared to be among the 27 people killed in a terrorist attack at a holiday resort in Tunisia, according to local radio reports.
Tunisian interior ministry spokesman Ali Aroui described the victims as mostly tourists but did not give any nationalities, but local radio reports said those killed in the resort of Sousse were mostly German and British.
The atrocity, along with a terror attack in France in which a man was decapitated, prompted Prime Minister David Cameron to announce that the Government's emergency Cobra committee would meet to discuss the situation.
Speaking at the conclusion of the European Council summit in Brussels, Mr Cameron said he had spoken to French president Francois Hollande to offer his support following the attack in Grenoble and would also speak to the Tunisian government following the attack there to offer "our solidarity in fighting this evil of terrorism".
He said: "This is a threat that faces all of us. These events have taken place today in Tunisia and in France but they can happen anywhere. We all face this threat.
"There will be a ministerially-chaired Cobra meeting, the Government's emergency committee, later on this afternoon to make sure we are doing everything we can to co-operate and co-ordinate with other countries and any information that we have we share with them in fighting this threat."
He added: "We have got to do all we can to help. That means co-operating on counter-terrorism, building our capacity on counter-terrorism, it means dealing with the threat at source whether that is Isil in Syria and Iraq or whether it is other extremist groups around the world.
"Perhaps more important than anything is poisonous radical narrative that is turning so many young minds and we have to combat it with everything we have."
The Prime Minister said that the West must stop "the poisoning of ... young minds" by Islamic State (IS) and other extremists.
"The people who do these things, they sometimes claim they do it in the name of Islam. They don't. Islam is a religion of peace. They do it in the name of a twisted and perverted ideology that we have to confront with everything that we have.
"We must stop the poisoning of these young minds in our country, in other European countries, and around the world."
One gunman was killed and another is being pursued following the attack in the town of Sousse, a popular holiday destination for Britons.
Thomas Cook, which has holidaymakers in the resort, said: "Thomas Cook has been advised of an incident that occurred earlier today in Sousse, Tunisia.
"At this time, details are not clear as to which property(ies) have been affected, with conflicting news reports.
"We are currently gathering information and will provide an update as soon as possible. Our teams on the ground are offering every support to our customers and their families in the area.
"We will continue to monitor the situation, working closely with the FCO and local authorities."
In Grenoble, French authorities have revealed that one of the suspected Islamic terrorists accused of carrying out the attack was known to police.
The victim's severed head was found stalked on a gate at the entrance of a gas factory in the town in south west France, with a message written on it, reportedly in Arabic.
French officials said two men in a car had earlier crashed into the factory site and ploughed into gas canisters, sparking an explosion, at around 10am local time.
Two other people were injured in the incident which president Hollande described as having "all the hallmarks of a terrorist attack".
The victim's torso was found close to the factory site, while two flags - one white and one black and both with Arabic inscriptions - were discovered at the scene, a security official said.
The victim has not yet been formally identified but local media reported that he was a transport manager who was at the factory for a delivery.
One of the suspected attackers - a 30-year-old man - has been arrested after he was detained by a firefighter.
French interior minister Bernard Cazeneuve said the suspect had been under investigation for possible ties to Islamic extremists in 2006, but surveillance was dropped in 2008.
A number of other people are in custody, he added.