(http://news.bbcimg.co.uk/media/images/75329000/jpg/_75329063_nigeriamaidugurigwozamandara4640614.jpg) Nigerian President Goodluck Jonathan declared a state of emergency in May 2013 in the three northern states where Boko Haram is most active - Borno, Adamawa and Yobe.
The militants retaliated by stepping up their bombing campaign in cities and raiding small towns and villages.
Many people have fled their homes - some over the borders to Cameroon, Chad and Niger.
Correspondents says since the kidnapping of the girls from their boarding school in Borno state on 14 April, the attacks have become an almost daily occurrence.
A new report by the Internal Displacement Monitoring Centre and the Norwegian Refugee Council says 3,300 people have been killed by Boko Haram this year alone.
The UK, the US, China and France are among those countries to have sent teams of experts and equipment to help to locate the girls.
(http://news.bbcimg.co.uk/media/images/74982000/jpg/_74982323_line976.jpg) Who are Boko Haram? (http://news.bbcimg.co.uk/media/images/75329000/jpg/_75329065_75007899.jpg) Boko Haram leader Abubakar Shekau has been designated a terrorist by the US government
- Founded in 2002
- Initially focused on opposing Western education - Boko Haram means "Western education is forbidden" in the Hausa language
- Launched military operations in 2009 to create Islamic state
- Thousands killed, mostly in north-eastern Nigeria - but also attacks on police and UN headquarters in capital, Abuja
- Some three million people affected
- Declared terrorist group by US in 2013
Who are Boko Haram? (http://www.theinfostride.com/news/world-africa-13809501)
Profile: Boko Haram leader Abubakar Shekau (http://www.theinfostride.com/news/world-africa-18020349)
Why Nigeria has not defeated Boko Haram (http://www.theinfostride.com/news/world-africa-27396702)
Source: BBC.co.uk