LAGOS (Reuters) - Amnesty International said on Friday that Nigeria's army last month killed at least 17 unarmed members of a group calling for secession from Africa's most populous nation, but the military dismissed the allegations as unfounded.

An army spokesman said Amnesty's accusations, the latest in a series of allegations of impropriety leveled against Nigeria's military in the last year, revealed a bias that undermined its credibility.

Amnesty's report -- which it said was based on details from eyewitnesses, morgues and hospitals -- says soldiers opened fire on members of the Indigenous people of Biafra (IPOB) and their supporters in the southeastern city of Onitsha during the build up to a march in late May.

The human rights group said the killings took place during a security operation in the early hours of the morning shortly before the march when the military raided homes and a church where IPOB members slept.

The army issued a statement in which it said troops had to "resort to self defense" after IPOB members attacked security agencies with "firearms" and various weapons including dynamite.

It said five members of IPOB were killed, eight wounded while nine were arrested.

"These efforts were in order to de-escalate the palpable tension as well as ward off the apparent threats to lives and property in the general area," it said.

Secessionist feeling has simmered in the southeast since the Biafra separatist rebellion tipped Nigeria into a 1967-70 civil war that killed an estimated 1 million people.

It flared up again last year after IPOB leader Nnamdi Kanu was arrested and detained on charges of criminal conspiracy and belonging to an illegal society. He remains in detention.

Amnesty said its investigation showed at least 17 people were killed and nearly 50 injured, adding that "the real number is likely to be higher".

"Information gathered by Amnesty International indicates that the deaths of supporters and members of IPOB was the consequence of excessive, and unnecessary use of force," said Amnesty, which urged the government to investigate.

The contents of the report were rejected by army spokesman Sani Usman. "The allegations are unfounded," he said.

The report is the latest in a series of accusations leveled at the army by Amnesty.

Last year, Amnesty said more than 8,000 people died in detention during a crackdown on Boko Haram and that soldiers killed hundreds of Shi'ite Muslims in the northern city of Zaria in December.

"Amnesty is losing credibility," added Usman, who accused the group of being biased against the army.

(Reporting by Alexis Akwagyiram; Editing by Toby Chopra)