The British Foreign Office has warned tourists that further attacks in Tunisia are possible, following an attack on a resort in the town of Sousse that killed at least 15 of the country's citizens.

On its website, the Foreign and Commonwealth Office (FCO) has updated its advice for tourists already in Tunisia, or who are planning to visit the North African country in the near future.

"Further terrorist attacks in Tunisia, including in tourist resorts, are possible, including by individuals who are unknown to the authorities and whose actions are inspired by terrorist groups via social media," the FCO wrote.

"You should be especially vigilant at this time and follow the advice of Tunisian security authorities and your tour operator."

The warning advises tourists against visiting some areas of the country including the Chaambi Mountain National Park area. In addition, the FCO advises against visiting other areas including areas south of, and including, the towns of Nefta, Douz, Médenine and Zarzis unless essential.

Of an estimated 20,000 Britons who were in Tunisia at the time of Friday's attack by lone gunman Seifeddine Yacoubi, around 2,500 are thought to have returned home, many on specially chartered planes organized by tour operators.

All trips to Tunisia for the next week have been cancelled and tour operators said anyone who wanted to cancel their trip would be refunded.

Although some Britons and tourists from other European countries including Ireland and Germany have decided to stay, so many are trying to leave the country they are having to queue outside the airport.

Ordinary Tunisians -- many of whom rushed to help guests during the massacre -- now face financial ruin. Many local people demonstrated in Sousse against the Islamist atrocity on Saturday evening, carrying flags including the Union Jack and banners proclaiming: "Sousse will never die."