Campaigners there are up to 150 acid attacks every year

Three sisters have suffered serious facial burns after two unidentified men on a motorbike threw acid at them in Pakistan's Balochistan province.

The sisters, aged between 14 and 20 years old, were attacked as they walked from Kalat city to Pandarani village - one is still in a serious condition.

Political activists held a protest in Kalat shortly after the attack.

There are no reliable statistics, but campaigners say there may be 150 acid attack victims in Pakistan each year.

The police named the girls as Fatima Bibi, 20, Saima Bibi, 16 and Sakina Bibi, 14.

They were taken to a government hospital in Kalat, but Fatima Bibi was later shifted to a hospital in the provincial capital, Quetta.

No arrests have been made as yet.

Two weeks ago, an unknown group - the Baloch Ghairatmand Group (the Honourable Baloch Group) - claimed responsibility for a similar attack on two women in a market in Dalbandin city.

The group had warned women to wear the hijab, the traditional Muslim headscarf, and not to visit markets unaccompanied by men from their families.

The attack was criticised by Balochistan's political leaders as well as armed rebel groups, who said it had been carried out by elements that wanted to push the Baloch people back in time.