Men riding a motorbike threw acid at two British teenage girls in Tanzania's semi-autonomous Zanzibar region, leaving them with facial, chest and back injuries, a senior police official said on Thursday.The Zanzibar government has offered a $6,170 reward for information on attackers who threw acid at two British Jewish women.Katie Gee and Kirstie Trup, both 18 and reportedly active members of the Federation for Zionist Youth (FZY), are from England's northern city of Manchester. According to police in the Indian Ocean archipelago, they were flown to Tanzania's commercial capital Dar es Salaam.Trup’s father, Marc Trup, a dentist in Hampstead, told the Telegraph: “We're trying to get them back as soon as possible. We're trying to organize a medivac flight." They had been volunteering at a local school in Zanzibar, an island that is popular with international tourists but suffered a wave of deadly protests last year as supporters of an Islamist group repeatedly clashed with the police.Britain is concerned about Wednesday's attack and is "in contact with the Tanzanian authorities," the Foreign Office said in a statement.A friend of Trup's, Oli Cohen, 21, told the Jewish Chronicle, "Katie was attacked two weeks ago by a Muslim woman for singing on Ramadan. She was shocked as it just came from out of the blue - but she wasn't scared enough to come home she stayed out there to finish her trip and volunteering.” The police, however, described the attack as "an isolated incident", refusing to link it to rising religious tension on the island between majority Muslims and its Christian population.The police said it was the first time visitors to the island had been attacked in this way, describing the incident as “alien” to both the police investigating and the victims. Officers had no idea as to the motive, they said."The attackers approached the girls as they were walking on a street at around 7:15 p.m. and threw acid at them," Zanzibar Urban West regional police commander Mkadam Khamis Mkadam told Reuters. "The incident occurred when the streets were deserted as most people were breaking their Ramadan fast."Television images showed one girl obviously in pain in the back of a car at the Zanzibar airport."The victims sustained facial, chest and back injuries from the acid attack," Mkadam said.According to the Jewish Chronicle, Gee's mother, Nicky was previously featured in the Chronicle as a supporter of the Israel-based Hazon Yeshaya Soup Kitchens charity. Friends of the victims took to Twitter to express their shock and support, the Chronicle reported. Sabrina Davies tweeted, "Hope @katiejgee and Kirstie make a speedy recovery, thinking of you both xxx." Louisa Green also tweeted, "Can't actually believe what I just read in the news... Absolutely horrific. Thoughts are with @katiejgee and Kirsty Xxx".Jemima Schav also contributed to the Twitter conversation, saying, "Really hoping @katiejgee is okay right now".The Britons were expected to fly home on Thursday.The attack came during the tourist season in the historic town and after a Zanzibar Muslim leader, Sheikh Fadhil Suleiman Soraga, was hospitalised with acid burns in a November attack.Two Christian leaders were killed early this year in separate attacks.A separatist group in Zanzibar, Uamsho (Awakening), is pushing for the archipelago to exit from its 1964 union with mainland Tanzania, which is ruled as a secular country. Uamsho wants to introduce Islamic Sharia law in Zanzibar.Supporters of the group have engaged in running street battles with the police in the past, but authorities have not linked the group with the attacks on Christian clerics.