United Nations Secretary-General Antonio Guterres called on Nigerian authorities to investigate allegations of systemic and coerced abortions reportedly perpetrated by the Nigerian army, UN spokesperson Stephane Dujarric said on Friday.

Reuters reported on Wednesday that the Nigerian Army has run a secret, systematic and illegal abortion programme in the country's northeast since at least 2013.

The programme involved terminating at least 10,000 pregnancies among women and girls, many of whom had been kidnapped and raped by Islamist militants, according to dozens of witness accounts and documentation reviewed by Reuters.

"The secretary-general takes note with concern of the allegations of systemic and coerced abortions reportedly perpetrated by the Nigerian Army against women and girls who had already been victimised by Boko Haram," Dujarric said in an email to Reuters.

He called for a thorough investigation and "immediate remedial actions and accountability measures," if such measures were necessary.

"We call on the Nigerian authorities to fully investigate these allegations and make sure there's accountability," Dujarric told reporters later on Friday.

A US State Department spokesperson said the Biden administration was "deeply troubled."

"Our embassy in Abuja is seeking further information, including from the government of Nigeria and stakeholders working in this space," the spokesperson said. "We have encouraged the government of Nigeria to take the allegations seriously and to conduct a thorough and transparent investigation, and we will continue to do so."

Nigeria's defence chief said on Thursday the military will not investigate the report, saying it was not true. The Nigerian government has not responded publicly to the story.

Nigeria's Information Minister Lai Mohammed could not immediately be reached for comment on the UN call for an investigation.

- Reuters