A Zanzibar police chief has said that five people have been questioned over an acid attack on two British teenagers, two of whom are being detained for further interrogation. Regional police commander Mkadam Khamis said the other three have been released.

Katie Gee and Kirstie Trup, both 18, were attacked by two men on a motorbike as they walked through the historic capital Stone Town on Wednesday night. The acid was splashed over their faces, chests, backs and hands.

The pair, who had been volunteering at an orphanage on the island, were taken to hospital in Dar es Salaam on the Tanzania mainland, and are due to be flown home today.

The young women, from north London, were enjoying the last week of a trip as volunteer teachers to the predominantly Muslim island when a corrosive substance was thrown at them in an apparently unprovoked attack.

The trip had been organised by i-to-i Travel, which is based in Tunbridge Wells, Kent, and runs gap year trips to parts of Africa, Asia, central and South America and Australia. Trips to teach in Zanzibar, based in Stone Town, start at £669 for two weeks, according to its website.

In a statement, i-to-i Travel said the pair were released from a hospital in the Tanzanian capital, Dar es Salaam, where they had been flown for medical treatment after the "acid attack".

"All our efforts remain focused on ensuring they are supported whilst assisting them and their relatives with the arrangements for their return home," it said.

The two women are due home today

The British Foreign Office travel advice for Tanzania warns that although most visits to the country are trouble-free, "violent and armed crime is increasing".

The advice says "Mugging, bag snatching (especially from passing cars) and robbery have increased throughout the country."

It adds "In Zanzibar, incidents have taken place in Stone Town and on popular tourist beaches."

Jakaya Kikwete, Tanzania's president, is reported to have visited the women at the Aga Khan Hospital in Dar es Salaam and promised that the men responsible for the "shameful" attack would be found.

Zanzibar is an archipelago of islands in the Indian Ocean around 22 miles off the mainland. The semi-autonomous region of Tanzania is predominantly Muslim and has been the scene of some religious violence in recent years.

Earlier this year two Christian leaders were killed and in November a cleric was treated in hospital after an acid attack. This is thought to be the first attack on tourists.