Tunisian authorities arrested suspects in relation to a beach massacre that killed 38 people in the country’s worst jihadist attack.

The attack claimed by the Islamic State (ISIL) terror group was the deadliest for Britain since the 2005 London bombings, with the UK death toll expected to total 30.

Local police arrested “a significant number” of people linked to lone gunman Seifeddine Rezgui, 23, who slew his victims with machine gun bullets fired from a Kalashnikov assault rifle hidden inside a beach umbrella.

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British Home Secretary Theresa May travelled on Monday to the resort of Port el Kantaoui south of Tunis, promising to fight extremism in the wake of the attack.

“We will be united in working together to defeat them but united also in working to defend our values,” Ms May said at a joint news conference with her German, French and Tunisian counterparts after visiting the scene of the killings.

Ms May and the German and French interior ministers, Thomas de Maiziere and Bernard Cazeneuve, joined Tunisian officials in laying a wreath in the sand near the Riu Imperial Marhaba Hotel where the attack happened.

A spokesperson for British Prime Minister David Cameron, meanwhile, said Britain had identified 18 of its nationals killed, but warned that the number may rise to “around 30”.

Tunisia says four other victims have been identified as being tourists from Germany, Portugal, Ireland and Belgium.

-with AAP, ABC