Reuters/Yola, Nigeria

A bomb killed at least seven and injured 20 on Friday at a camp in northeast Nigeria for people who have fled Boko Haram, state emergency agency officials said.

There was no immediate claim of responsibility. About 2.1 million people have been displaced and thousands killed in a six-year-old campaign by the jihadist group to carve out an Islamist state in Nigeria's northeast.

The group has killed around 800 people in bombings and shootings since President Muhammadu Buhari took office in May, vowing to crush the militant Islamists.

The blast, the first such attack on a camp for internally displaced persons (IDPs), was at a school in a hamlet on the outskirts of the Adamawa state capital, Yola.

"So far seven persons lost their lives and 20 persons were injured in the bomb blast that occurred in Malkohi Internally Displaced Persons camp," said Sani Datti, a spokesman for the National Emergency Management Agency (NEMA).

"Among the injured, seven were treated and discharged while 13 persons including four NEMA officials are still receiving treatment," he added.

Datti said the bomb was planted in a tent and went off at around 10:50am (0950 GMT).

Most IDPs are living in government-run camps, with host families or with religious organisations in the city of Maiduguri in Borno state, the heartland of the insurgency.

Adamawa state hosted over 250,000 people earlier this year in and around Yola and also received many women and children freed by Nigerian troops from Boko Haram camps.

Several cities in Adamawa also took in thousands of Nigerian refugees from Boko Haram attacks who were evicted from Cameroon last month and continue to flow in.

Earlier this year, joint offensives by troops from Nigeria, Niger and Chad led to the reclamation of territory that had been seized by the militant Islamist group.

Now, dispersed across Borno state and other parts of the country, Boko Haram members have reverted to guerrilla tactics and frequently target places of worship, markets and checkpoints.