Gunmen shot dead at least 30 travellers in an attack in Nigeria's southeastern Imo state, Amnesty International said on Friday, raising fresh concerns about violence in a region rife with insecurity.

More than 20 vehicles and trucks were set ablaze by the attackers, who were suspected to be members of the banned separatist Indigenous People of Biafra (IPOB), Amnesty said in a post on X.

Imo police spokesperson Henry Okoye confirmed the attack occurred in Thursday's early hours, but declined comment on the number of fatalities. One of the assailants was killed by the police, Okoye told Reuters on Friday.

A police statement said the gunmen, operating in three groups, barricaded the highway at about 0400 GMT and shot sporadically before setting vehicles ablaze.

"A full-scale search and cordon operation is currently underway, with security operatives combing nearby forests and surrounding areas where the suspects are believed to be hiding," the police said in the statement.

IPOB campaigns for the secession of southeastern Nigeria, where the majority belong to the Igbo ethnic group. Nigerian authorities have labelled IPOB a terrorist organisation.

Civil war engulfed the Biafra region in the late 1960s, killing more than 1 million people.

Thursday's attack coincided with a visit by President Bola Tinubu to the region, occurring in the same week that IPOB leader Nnamdi Kanu appeared in federal court where he is facing trial on terrorism charges.

Amnesty called on Nigerian authorities to investigate the attack and bring the perpetrators to justice.

Reuters