The US embassy in Nigeria on Thursday warned Americans about the threat of attacks by extremist groups following the violence in Libya and Egypt, urging extra caution.
The "emergency message" issued to Americans living in Africa's most populous country came after militants attacked the US consulate in Benghazi, killing the US ambassador and three other Americans, hours after a mob attacked the US embassy in Cairo.
"Extremists may attempt to target US citizens and other Westerners in Nigeria," the embassy said in a statement.
It noted that radical groups in Nigeria have killed hundreds in recent months in attacks on a range of targets.
"The situation in Nigeria remains fluid and unpredictable," the statement said, urging Americans "to consider their personal security."
On Wednesday, Nigeria's police chief ordered "24-hour water-tight security" at all foreign embassies.
The attacks in Cairo and Benghazi were initially believed to have been perpetrated by mobs outraged over an amateur Internet film made in America that insulted Islam but US officials later said the Benghazi attack might have been planned.
Religiously-linked violence has killed thousands of Nigerians in recent years. Radical Islamist group Boko Haram is blamed for more than 1,400 deaths since 2010, while sectarian clashes between Muslim and Christian communities have killed thousands more.