The state department has also authorised non-emergency embassy staff to leave on security grounds.
People evicted to build Nigeria's capital accuse the government of failing to fulfill its promises to them.
Prince Harry and Meghan Markle have been welcomed by a charity which helps service personnel in Nigeria.
The Duchess of Sussex cheered on her husband from the side of the court.
A ransom was paid to free the six girls who were kidnapped earlier this month, an uncle tells the BBC.
The girls were kidnapped from their home in Abuja, along with another sister who was later killed.
The defence minister hits out at those paying ransoms to kidnappers after the abduction of six sisters.
Long queues have formed at polling stations in Nigeria as millions vote in the most competitive presidential election since military rule ended.
President Buhari vowed to defeat Boko Haram when elected in 2015. Is north-east Nigeria safe now?
