At least five Britons have been confirmed dead in a terrorist attack on a Tunisian holiday resort, with the death toll expected to rise.
Foreign Secretary Philip Hammond said because Sousse is popular with holidaymakers from the UK "we have to assume that a high proportion of those killed and injured will have been British".
An Irish mother-of-two who was shot dead was named locally as Lorna Carty, from Robinstown, Co Meath.
Family friends said she had taken her husband on holiday to help him recover from heart surgery, and she was believed to have gone to the beach by herself when the gunman went on the rampage.
She was a nurse in a GP surgery in Navan, aged in her 50s, and had a son and daughter. Her husband Declan, a dairy farmer, was said to be uninjured but "absolutely distraught".
A total of 37 people died in the attack when a young man pulled out a Kalashnikov hidden in a beach parasol and opened fire on sunbathers.
The worst such attack in Tunisia's history came on the same day a man was found decapitated after an attack by suspected Islamic extremists on a French factory and a Shiite mosque in Kuwait was bombed killing at least 25 people.
Although the attacks do not appear to be directly linked they come after the so-called Islamic State called for their followers "to make Ramadan a month of calamities for the nonbelievers".
Tunisia's Health Ministry confirmed that those killed included Britons, Tunisians, Germans and Belgians. Thirty six others have been wounded.
Bodies covered in blankets were strewn across the beach and medical staff used sun loungers as stretchers to carry away the dead and injured.
Despite initially believing there was more than one gunman, the Tunisian authorities later said the killer was thought to be a young student who had not previously been known to their security services.
His killing spree on the beach at the Riu Bellevue Park ended when he was shot dead by police. A bomb was found on his body.
British tourist Ellie Makin, from Ripon, North Yorkshire, who was on holiday with her friend Debbie Horsfall from Huddersfield, said the gunman was next to them on a sunbed.
She told ITV News: "He was to the right of me because we were on the last sunbed on the beach. All of a sudden I got up anyway and just happened to look right and all I saw was a gun and an umbrella being dropped.
"Then he started firing to the right hand side of us. If he had fired to the left I don't know what would have happened, but we were very lucky."
Olivia Leathley, 24, was with her boyfriend Mike Jones in her room at the hotel when she heard "loud bangs" from the beach.
They ran to the lobby where they found a woman whose husband had been shot in the stomach in front of her.
Miss Leathley said: "All she said was that he'd been shot and that he was there bleeding on the beach and he was just saying, 'I love you I love you', and then his eyes rolled back into his head. But at that point she was dragged away by people who were trying to keep her safe."
She was thought to be referring to Saera Wilson, who later told the BBC that her fiance Matthew James used his body as a human shield.
She said gas engineer Mr James, 30, from Trehafod, near Pontypridd, was hit in the shoulder, chest and hip.
Speaking from the hospital where Mr James is being treated, Ms Wilson said: "He took a bullet for me. I owe him my life because he threw himself in front of me when the shooting started.
"He was covered in blood from the shots, but he just told me to run away.
"He told me: 'I love you babe. But just go - tell our children that their daddy loves them'.
"It was the bravest thing I've ever known."
Those killed are more likely to be foreign as the local Muslim population is less likely to go to the beach during the holy month of Ramadan.
Tension has been high in Tunisia since an attack on the National Bardo Museum in March which killed 22 people, mostly foreign tourists including a Briton.
A suicide bomber blew himself up in a failed attack on the beach in Sousse in October 2013, while 21 people lost their lives in an attack in the country earlier this year.
Tunisia has undergone unprecedented social and political change since the 2011 uprisings and the Foreign Office says there is a ''high'' threat from terrorism.
Sousse is on the east coast, about 87 miles (140km) south of the capital, Tunis. Around 1.2 million tourists visit Sousse every year, drawn by the hotels, sandy beaches and culture.
Speaking after chairing a meeting of the Government's Cobra committee and following talks with tour operators, a sombre Mr Hammond confirmed the British fatalities.
He told reporters at the Foreign Office: "I would like to offer my condolences to the families and friends of the victims of these three shocking terrorist incidents in Tunisia, Kuwait and France.
"I have chaired a Cobra meeting this afternoon to look at our response to all three of these incidents and there will be a further Cobra meeting tomorrow morning.
"Turning to the incident in Tunisia which is of most direct concern to people in this country because of the number of Britons involved, it is clear that there have been a number of people killed.
"The situation on the ground is still somewhat confused and we can't be sure exactly how many, but because of the nature of the composition of the tourist population in this part of Tunisia we have to assume that a high proportion of those killed and injured will have been British.
"We have had reports from families of those involved in the incident that allow us to confirm that at least five Britons have been killed in this incident, but I should warn that we must expect that there will be more reports of fatalities as we establish the detail on the ground."
A consular team is in Sousse, with two rapid response teams travelling to the region overnight, Mr Hammond said.
"We are working with the Tunisian authorities to understand what they are doing in response to this incident and around the tightening of security in this area.
"We are talking to other national authorities - particularly the German authorities who had large numbers of tourists in the area - and I have just finished a conference call with the tour operators who operate the majority of flights into this area."
David Cameron, who condemned the sickening attacks in Tunisia, France and Kuwait, said the families of the British victims would be supported.
He said: "My heart is also with the families of all the British victims of the Tunisian terror attack. We are to help and support them."