MAIDUGURI, Nigeria (Reuters) - Boko Haram insurgents launched their biggest attack on the northeast Nigerian city of Maiduguri in 18 months on Wednesday night, the eve of a visit by Acting President Yemi Osinbajo to war refugees sheltering there.

Police said that 14 people were killed before government troops beat back the raid.

Maiduguri is the center of the eight-year-old fight against Boko Haram, which has been trying to set up an Islamic caliphate in the northeast.

The fighters attacked the city’s suburbs with anti-aircraft guns and several suicide bombers, said Damian Chukwu, police commissioner of Borno State, of which Maiduguri is the capital.

“A total of 13 people were killed in the multiple explosions with 24 persons injured, while one person died in the attack (shooting),” he told reporters.

Osinbajo went ahead with his visit to Maiduguri, planned prior to the attack, launching a government food aid initiative to distribute 30,000 metric tonnes of grains to people displaced by the insurgency, his spokesman Laolu Akande said.

President Muhammadu Buhari handed power to Osinbajo after going to Britain on medical leave on May 7.

Aid workers and Reuters witnesses reported explosions and heavy gunfire for at least 45 minutes in the southeastern and southwestern outskirts of the city. Thousands of civilians fled the fighting, according to Reuters witnesses.

The police commissioner said several buildings were set on fire but the military repulsed the fighters after an hour.

The raid took place six months after Buhari said Boko Haram had “technically” been defeated by a military campaign that had pushed many insurgents deep into the remote Sambisa forest, near the border with Cameroon.

More than 20,000 people have been killed in Boko Haram’s campaign to establish a caliphate in the Lake Chad basin. A further 2.7 million have been displaced, creating one of the world’s largest humanitarian emergencies.

Despite the military’s success in liberating cities and towns, much of Borno remains off-limits, hampering efforts to deliver food aid to nearly 1.5 million people believed to be on the brink of famine.

The government food program launched by Osinbajo seeks to distribute grains to 1.8 million people delivered quarterly, his office said in an emailed statement.

The acting president, speaking in Maiduguri, said a “comprehensive livelihood and support program” would be launched by the government within weeks.

A United Nations official on Wednesday said the U.N. World Food Programme (WFP) has had to scale back plans for emergency feeding of 400,000 people in the region due to funding shortfalls.