Up to half the food aid meant for people who have fled Nigeria's Islamist insurgency has reportedly not been delivered, the government says.

It described it as a "diversion of relief materials", which correspondents say is a euphemism for theft.

A statement from the acting president's office added that security was being beefed up to protect the deliveries.

As a result of Boko Haram violence some 8.5 million Nigerians in the north-east need life-saving aid, officials say.

Poor rains have exacerbated a problem caused by fighting with Boko Haram Islamist militants, which has forced hundreds of thousands of people to flee their homes.

People in north-east Nigeria are also facing a possible famine, the UN said in March.

In the latest suspected Boko Haram attack on Sunday, 12 people were killed by five suicide bombers in a village near Maiduguri, the capital of Borno state.

The statement from acting President Yemi Osinbajo said aid going missing had "dogged food delivery" and then cited reports saying that more than 50 lorries out of every 100 sent to the north-east never reach their destination.

It does not say what has happened to the diverted food, but in May two Nigerian officials were jailed for selling food aid.

Last week, Nigeria apologised to Saudi Arabia after 200 tonnes of dates the kingdom sent as a Ramadan gift were found on sale in local markets.

Mr Osinbajo said that the latest consignment of aid which is making its way to the north-east is being protected by more than a thousand soldiers.