Gunmen who attacked a train in northwest Nigeria in March have freed 11 of their hostages in return for the release of their own children, security sources told AFP on Sunday.

The captives were freed on Saturday, after weeks of negotiations with the authorities, in return for eight of the assailants' children, who had been seized by the army, the sources said.

The attackers had issued a video in May threatening to kill the hostages if an agreement was not found.

"Eleven of the passengers being held by the gunmen were released yesterday," one source said.

"They were swapped with eight children of the attackers who were taken in a previous (army) raid," the source added.

"The agreement was for them to free all the female hostages in their custody in exchange for the children but they later reneged and released six women and five men," a second security source said.

Both sources asked not to be identified because they were not authorised to speak on the issue.

The hostages were taken to Kaduna city in the northwest on Saturday evening before being transferred to the capital, Abuja, for medical attention, the sources added.

On March 28, armed men blew up a train travelling between Abuja to Kaduna and opened fire, killing eight people, wounding 26 and taking an unspecified number of passengers hostage.

A week later, they freed one hostage -- a top bank executive -- as a goodwill gesture for Ramadan, the Muslim holy month of fasting, because of his "advanced age".

In late May, the group posted a video of other hostages, threatening to execute all their captives if the government refused to meet their demands within in a week. They withdrew the threat several days later.

Security sources believe Ansaru jihadists cooperated with an armed criminal gang to stage the attack, which marked a major escalation in violence in northwest Nigeria.

Ansaru, which split from the Islamist group Boko Haram in 2012, is believed to be active in central Kogi state, and around Kaduna and Abuja.

Gangs of bandits with no ideological or religious motives are known to kidnap for ransom in northwest and central Nigeria.

But there is concern among local authorities and analysts that some are forging alliances with Islamist militants for pragmatic and financial reasons.