At least 30 people were killed when gunmen attacked travellers in Nigeria?s southeastern Imo State, according to Amnesty International.

The human rights NGO reported on Friday that more than 20 commuter vehicles and trucks were set ablaze during a ?vicious attack? along the Okigwe-Owerri road the day before.

The attackers are suspected of being members of the banned separatist group, Indigenous People of Biafra (IPOB), Amnesty said.

?The attack ? shows callous disregard for the sanctity of life. The gunmen blocked Okigwe-Owerri highway and areas of Umuna in Onuimo LGA unleashed violence in an utter show of impunity,? Amnesty wrote on X.

?The Nigerian authorities must immediately and transparently investigate this attack and ensure that the actual perpetrators are brought to justice. International law requires the Nigerian government to promptly investigate unlawful killings to bring perpetrators to justice,? it added.

Imo police spokesperson, Henry Okoye, confirmed that the attack took place in the early hours of Thursday, but declined to comment on the death toll.

On Friday, Okoye told the Reuters news agency that one of the assailants had been killed by the police.

According to a police statement, the gunmen, operating in three groups, blocked the highway at 04:00 GMT and shot sporadically.

?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.

Nigeria?s Premium Times also reported that the police commissioner of Imo State, Aboki Danjuma, led a joint tactical team of security operatives to the scene.

According to Danjuma, the joint security team comprised personnel from Nigeria?s army, the police, and the State Security Service.

The IPOB, which the Nigerian government has classed as an outlawed organisation, has been campaigning for the secession of southeastern Nigeria, where the majority of people belong to the Igbo ethnic group.

Following the late 1960s civil war that engulfed the Biafra region, killing more than one million people, separatist groups have continued to push for an independent state from the southeast and in some parts of the south of Nigeria (Al Jazeera, excluding headline)

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