Nigeria has provoked international outrage after President Goodluck Jonathan ratified a controversial bill outlawing gay marriage and same-sex unions under threat of imprisonment.
Secretary of State John Kerry said the US is "deeply concerned" after Jonathan signed the Same Sex Marriage (Prohibition) Bill 2013, which proposes up to 14 years in jail for law-breakers.
The veteran gay rights campaigner Peter Tatchell described it as "one of the world's most homophobic laws" while advocacy groups called it "tragic" and a backward step in the fight for equality.
Jonathan's spokesman Reuben Abati earlier confirmed that the head of state had signed the bill earlier this month after it was approved by parliament last year.
But he dismissed global concerns, saying the law was consistent with the attitudes of most people towards homosexuality in the highly religious west African nation.
"More than 90 per cent of Nigerians are opposed to same sex marriage. So, the law is in line with our cultural and religious beliefs as a people," he added.
The anti-gay marriage law follows similar legislation in Uganda that was condemned by US President Barack Obama as "odious" and compared to apartheid by South African peace icon Archbishop Desmond Tutu.
The bill, passed by Uganda's parliament but yet to be ratified by President Yoweri Museveni, had previously contained a clause advocating the death penalty.
Homosexuality is also illegal in Nigeria's eastern neighbour, Cameroon, and punishable by up to five years in prison.
The influence of evangelical Christianity is strong in all three countries while Nigeria is almost evenly split between a largely Christian south and majority Muslim north.
