Blasts have killed at least 20 people in northeast Nigeria, police said, while residents said the toll was even higher in the largest attack for weeks in a region blighted by the Islamist militant Boko Haram insurgency.

In an emailed statement issued on Sunday, the police said 20 people had been killed and 48 injured. Two residents however said they had counted at least 31 dead.

The explosions occurred in the Damboa local government area in the south of Borno state, on Saturday at around 8:30 p.m. (1930 GMT). Witnesses spoke of at least one rocket attack.

Security in Nigeria has become a major challenge for President Muhammadu Buhari, a former military ruler whose 2015 election win was largely due to his vow to crush Boko Haram. It is a highly charged issue in the run-up to a poll due to take place in February 2019. Buhari has said he will seek a second term.

Borno is the state worst hit by the insurgency, aimed at creating an Islamic caliphate in northeast Nigeria, which has killed more than 30,000 people and forced over 2 million to flee their homes since 2009.

The blasts occurred in the Shuwari and Abachari districts, about 90 km (55 miles) from state capital Maiduguri. “It has destroyed our houses. We have also counted 31 innocent people including children and elderly killed in the attack,” said local resident Modu Usman, son of a community leader. There was no claim of responsibility for the blasts.

“Investigation is ongoing to unravel the nature of the attack,” said Edet Okon, a spokesman for Borno police. The last large attack in the northeast occurred in early May when at least 20 people were killed in Adamawa state, which borders Borno.