Two young British women were attacked by acid in Zanzibar, in what is believed to be the first such assault on foreigners on the fabled archipelago which is now a semi-autonomous part of the East African nation of Tanzania. Katie Gee and Kirstie Trup, both 18-year-olds from North London, were working as volunteer teachers in Zanzibar when two men riding on a passing moped threw acid on their faces, chests and hands on the streets of historic Stone Town in the old part of Zanzibar City, the capital, on Wednesday.

The girls were flown to a hospital in Dar es Salaam on the Tanzanian mainland and released on Thursday morning. The president of Tanzania himself, Jakaya Kikwete, reportedly visited the young women at the Aga Khan Hospital in Dar es Salaam and vowed to catch the culprits behind this "shameful" attack.

The motive for such an unusual incident remains a mystery. Typically, acid attacks occur on the Indian subcontinent and involved a jealous boyfriend, husband or suitor who takes revenge on a woman for such perceived insult. In such cases, the victims are physically and psychologically scarred for life “Investigations are on until we apprehend the criminals," Zanzibar’s deputy police commissioner Mkadam Khamis told the Agence France Presse news agency. "Police in Zanzibar have launched a manhunt, and we ask for public assistance in identifying the attackers."

The British girls, who are not suffering life-threatening injuries, are expected to return home to the United Kingdom on Thursday night. Katie’s mother Nicky told Britain’s Sky News that her daughter’s face and body were burned by the acid. "Both families are extremely upset and distressed at this completely unprovoked attack on their lovely daughters who had only gone to Zanzibar with good intention,” the girls’ mothers said in a statement. "We appreciate all the interest and support we have received from the media but we would ask that we are left alone until we have been reunited with our daughters."

Katie and Kirstie worked for a charity called Art in Tanzania, which, according to its website, was founded in 2001 and supports “local artists who did not have the resources to develop their talent.” Art in Tanzania is part of the UNICEF Children Agenda Tanzania program. Kari Korhonen, one of the principals of Art in Tanzania, told the BBC the "ladies are OK considering the seriousness of this type of case. We have been operating as NGO some 10 years and this is the first serious incident."

Zanzibar, which is predominantly Muslim, has in the past asked western and foreign tourists to abide by local customs, particularly with respect to dress in public, even on beaches. However since the local economy depends heavily on tourism, such edicts are usually not too strident. “Tourism is the strongest pillar of our economy, so if we do such acts we are killing our economy, and our livelihoods in general,” warned Tanzania's minister of information, tourism, culture and sports, Said Ali Mbarouk. "And I beg our nationals, this is not something they should be doing.  So it is not an honorable thing to do, it's a bad thing and it should be condemned by all citizens of Zanzibar."

According to the UK Foreign Office, some 75,000 Britons travel to Zanzibar and Tanzania every year, with few untoward incidents. But it cautions that "violent and armed crime is increasing" and that "there is an underlying threat from terrorism". "Mugging, bag snatching [especially from passing cars] and robbery have increased throughout the country," FCO further warned.

In recent months, Zanzibar has witnessed a number of violent attacks on religious figures, including a Catholic priest and a Muslim cleric, but not against foreigners. Religious tensions occasionally flare up, although Christians accounts for only a tiny 3 percent of the population.

However, a report in the British tabloid Mirror noted that both girls are of Jewish descent, raising a possible motive for the attack. The girls visited Israel in 2011 in a trip organized by the Federation of Zionist Youth (FZY). The FZY's director in Britain Harry Jardine said: "They spent a month in Israel volunteering, touring the country and learning the history of the area. Katie stayed involved but it was not a huge involvement, she took part in our leadership program. She is an engaged, intelligent, very sunny girl - an absolute pleasure to meet. I spent a month away with her and there was never any issue. When I heard the news I was shocked and I would like to wish the girls a speedy recovery."

Also, a mutual friend of the girls, named Oli Cohen, 21, told the Telegraph newspaper that this was not the first incident they endured in Zanzibar. "Katie was attacked two weeks ago by a Muslim woman for singing during Ramadan,” Cohen said. "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."