More than 2,000 prisoners have escaped over the past five years in Nigeria, officials say, mainly as a result of attacks on jails by the Islamist militant Boko Haram group.
They say that dozens of prison staff have been killed during the raids.
Last month hundreds of prisoners escaped when jihadists overran Mubi town. This week they blew a hole in a jail in Kogi State to free dozens more.
Almost 500 prisoners escaped in the city of Maiduguri in 2009.
A total of 2,251 prisoners have got away and most of them are still at large, officials say.
Figures released by the Nigerian Prisons Service show that as of 30 June there were nearly 57,000 male and female prisoners in 239 jails.
The BBC's Will Ross in Lagos says that the jihadists have recently attacked a French cement plant in northern Nigeria and seized a large supply of dynamite - so more jail breaks may be on the way.
Boko Haram has caused havoc in Africa's most populous country through a wave of bombings, assassinations and abductions.
It is fighting to overthrow the government and create an Islamic state.
Boko Haram regards the Nigerian state as being run by non-believers, even when the country had a Muslim president.
Our correspondent says rampant insecurity is increasingly worrying for Nigerians who wonder how the militants will ever be stopped.
There are many reports of soldiers fleeing rather than defending towns and villages as the insurgents continue to capture territory where they impose their own strict version of Islamic law.