Indian prosecutors said Tuesday they have formally charged eight men over the rape of an elderly Catholic nun that shocked the country earlier this year.

The nun, who is in her 70s, needed surgery after the attack in March when a gang of robbers broke into the convent school where she lived in the town of Ranaghat in eastern West Bengal state.

Six men, all Bangladeshi nationals, are in custody and another two suspects were charged in absentia.

The charges were filed on Monday and include robbery with attempt to cause death or grievous harm, criminal conspiracy and gang rape, the additional public prosecutor with the Ranaghat court, Pradip Kumar Pramanik, told AFP.

The prosecutor said further tests were needed to determine whether one or more people had committed the rape on the nun.

Indian law states that where a woman is raped by one or more people "acting in furtherance of their common intention", each can be deemed to have committed gang rape.

Police have said they identified all eight people who broke into the Convent of Jesus and Mary High School using CCTV footage.

One had been hired by the nuns to do construction work.

India is home to tens of thousands of Bangladeshis who officials say cross illegally into the country, mainly through borders in West Bengal, Tripura and Assam.

The two countries share a 4,000-kilometre (2,500-mile) border, part of which has been fenced by India in a bid to prevent illegal immigrants entering.

The immigration issue has driven a wedge between northeast India's Bengali-speaking population and local tribal and ethnic inhabitants, with Muslims bearing the brunt of decades of mistrust.