The RAF Sentinel R1 was deployed as part of a global effort to locate the girls. Photo: BBC

It has been forced to land in Senegal for repairs, the Ministry of Defence (MoD) said.

The plane left the UK on Sunday, joining an international effort to secure the release of the girls.

The Boko Haram group captured the girls from their boarding school in Chibok town, north-east Nigeria, in April.

In other developments:

• The police say they are reviewing security at boarding schools across the country
• The US on Monday said it had agreed to share some of its intelligence, including aerial imagery, with Nigeria's government in order to bolster the search for the girls
• The Senate is due to vote on President Goodluck Jonathan's request to extend the state of emergency imposed in three north-eastern states - it has already been approved by the lower house of parliament

'State-of-the-art'

The US military is flying manned and unmanned surveillance aircraft over Nigeria.

Israel says it has sent intelligence experts and specialists in hostage negotiations to help with the rescue effort.

Boko Haram has carried out a spate of bombings in north and central Nigeria

A video emerged last week showing about 130 of the girls wearing hijabs and reciting Koranic verses

The US had initially refused to share intelligence with Nigeria because of corruption within its military and fears that it had been infiltrated by Boko Haram, according to a Pentagon spokesman quoted on military.com.

The BBC's Will Ross in Nigeria says the plane's breakdown is a set-back for the UK government which promised to do all it can to help with the search operation.

The MoD said the "state-of-the-art surveillance jet" would operate from Ghana, and would join US aircraft in attempting to locate the girls.

It said the jet was "capable of flying for significant periods of time at high altitude" and was fitted with a "dual mode radar", which could "locate moving targets and offer radar imagery".

Boko Haram says it is prepared to free some of the girls in exchange for the release of its fighters and relatives being held by Nigeria's security forces.

Nigeria's government says it is prepared to hold talks with Boko Haram, but has reportedly ruled out a prisoner swap.

On Sunday, a suicide blast in a street full of bars and restaurants in a mainly Christian area of the northern city of Kano killed four people.

Officials blamed Boko Haram for the explosion.

Mr Jonathan declared the state of emergency a year ago in Borno, Adamawa and Yobe, where Boko Haram has been most active.

Although thousands of extra soldiers have been deployed to the area, the violence has continued, leading some north-eastern Senators to oppose the president's request to extend the state of emergency.

More than 1,000 people have been killed in attacks by the group this year.

African leaders meeting in Paris at the weekend agreed to wage "war" on Boko Haram, pledging to share intelligence and co-ordinate action against the group.