Two suicide bombers have attacked a town in north-eastern Nigeria only hours after the country's army chief urged displaced residents to return home because it was safe.

The blasts hit the town of Damboa in Borno state on Saturday evening and residents say at least 31 people died.

The explosions were followed up by rockets fired from outside the town.

Boko Haram militants are suspected. Army chief Lt Gen Tukur Buratai had said they were no longer a threat.

"Let me use this opportunity to call on the good people of northern Borno... to return to their communities which have long been liberated by our gallant troops," he said at an inauguration ceremony for gunboats earlier on Saturday.

A four-month military operation started on 1 May to expel Boko Haram insurgents from northern Borno and the Lake Chad region.

No group has said it carried out Saturday evening's attacks but a militia leader speaking to AFP, Babakura Kolo, said they bore the hallmarks of Boko Haram, a jihadist group that wants to establish a caliphate in northern Nigeria.

Officials said at least 20 people died in the attacks but residents said they had counted the dead and an anonymous local official confirmed the toll.

"It has destroyed our houses. We have also counted 31 innocent people including children and elderly killed in the attack," local resident Modu Usman, son of a community leader, told Reuters news agency.

More than 40 people were injured in the attacks, which were aimed at people celebrating the Eid al-Fitr holiday in the Shuwari and Abachari districts of the town.

The rocket attacks appear to have caused most of the casualties, a local official said.

The UN says 1.7 million people have been forced from their homes due to the Boko Haram conflict, which is now in its ninth year.

Boko Haram uses suicide bombers, often young girls, to target civilians and soldiers.

In one of the most recent attacks, bombers killed dozens of people in and around a mosque in the town of Mubi.

Despite the ongoing threat of suicide bombings, the security situation in north-east Nigeria has improved, says BBC Africa Editor Will Ross.

But there will be some scepticism about calls to return home, our correspondent adds. Previous promises that it is safe because the jihadists have been defeated have proved to be premature.