Congress had delayed the sale over concerns about the Nigerian army's commitment to protect civilians as it battles a jihadist uprising in the northwest, as well as the bloody repression of protests against police violence in 2020.

But President Joe Biden's administration approved the sale of the 12 Viper attack helicopters last year, saying the equipment would promote security in Africa's most populous country.

Representatives Chris Smith, a Republican active on human rights and strong opponent of abortion, and Sara Jacobs, a Democrat who formerly worked for the State Department, said that Nigeria's armed forces had a consistent record of abuses and that past aid had done little to boost security.

"Therefore, we believe continuing to move forward with the nearly $1 billion arms sale would be highly inappropriate and we urge the administration to rescind it," they wrote in a letter to Biden this week.

An investigation by Reuters last year found that Nigeria's military has conducted a years-long illicit program to carry out abortions among victims of jihadists, with some girls and women who refuse being beaten, held at gunpoint or drugged.

The military denied the report. The US lawmakers' call comes amid sporadic violence ahead of February 25 elections in Nigeria.

A State Department spokesperson, asked about the letter, said that the United States vets Nigerian forces before they receive US assistance.

"The Department does not provide assistance to a security force unit if there is credible information indicating the unit has committed a gross violation of human rights," she said.

"We consistently raise concerns about credible allegations of human rights violations and abuses at the highest levels and urge the Nigerian government to thoroughly and transparently investigate and hold to account those responsible for wrongdoing."