All British nationals injured in the terror attack on a Tunisian beach will be returned to the UK for treatment within the next 24 hours, Downing Street said, as the official toll of UK deaths rose to 18, ITV reported.

Officials confirmed that the total number of Britons killed when a gunman opened fire on sunbathers in the resort of Sousse is expected to reach "around 30", and said that the UK was pressing the Tunisian authorities to ensure swift access for British experts who have been sent to help identify remaining bodies.

Four of those needing treatment are being flown back to the UK on board an RAF C17 plane with "medevac" teams experienced at bringing injured service personnel back from operations overseas, said Prime Minister David Cameron's official spokeswoman.

The developments came as Home Secretary Theresa May visited the scene of the massacre to lay flowers and observe a period of silence.

May and Foreign Office minister Tobias Ellwood have traveled to the country to pay their respects and take part in talks with the Tunisian government on addressing the extremist threat.

May laid flowers and observed a period of silence in the Sousse resort.

Speaking at a news conference in Tunisia, she described the killings as "a despicable act of cruelty".

Standing alongside ministers from Tunisia, France and Germany, she said they were taking a united stand against acts of terror.

A total of 38 people perished after a gunman opened fire on a beach in the resort on Friday.