Hollywood actress Angelina Jolie, who campaigns against sexual violence in war zones, has said a "culture of impunity" is to blame for the abductions of hundreds of Nigerian schoolgirls.

Islamist group Boko Haram took the 276 girls from a boarding school in the village of Chibok, in Borno state, north Nigeria, on April 14th.

Abubakar Shekau, the group's leader, has threatened to sell the girls "on the market" and some of those kidnapped have reportedly already been trafficked to neighbouring Chad and Cameroon.

Speaking at the London premiere of the Disney film 'Maleficent', Jolie said "These men thought that they can get away with this, that they could abuse them in such a way, sell them, rape them, take them as property, because so many people have gotten away with this in the past because of this culture of impunity".

She added that the international community must "make sure this stops happening and that this is not something that people feel they can get away with, because right now it is and so it's awful that it's gotten to this point".

The American actress - who is also a special envoy for the UN High Commissioner for Refugees - will co-chair the Global Summit to End Sexual Violence in Conflict in London in June, along with the British Foreign Secretary William Hague.

The pair recently attended a conference on sexual violence in conflict in Sarajevo together and hailed Bosnia's decision to include rape prevention in military training as "groundbreaking".

All governments who have endorsed the UN Declaration of Commitments to End Sexual Violence in Conflict will be invited to the summit.