U.S. and other Western installations are at risk of terrorist attack in Africa's most-populous nation, and U.S. citizens should exercise vigilance in the West African nation, the U.S. Embassy said Thursday.

Nigeria, which has never suffered an attack by an international terrorist organization, is a top supplier of foreign oil for the United States, which is seeking alternative sources of crude amid turmoil in the Middle East.

"The U.S. Mission in Nigeria has received information that U.S. and other Western interests in Nigeria are currently at risk for terrorist attacks," the embassy said in a consular statement sent via e-mail to U.S. citizens in Nigeria.

"Potential targets include official and commercial installations in Abuja and Lagos," it said, without elaborating.

Embassy officials couldn't immediately be reached for comment. The statement didn't indicate which terror organization might be behind any attacks in Nigeria, a country of about 140 million citizens roughly split between Muslims and Christians.

In a statement attributed to Usama Bin Laden in recent years, Nigeria was targeted for liberation by Al Qaeda. Nigeria's president at that time was a Christian, while current President Umaru Yar'Adua is a Muslim who was governor of one the 12 states across the north where the Islamic civil code known as Shariah was installed in 2000.

Nigeria is a secular nation where opinion polls consistently show widespread admiration of America and the country's military and civilian administrations have generally been considered pro-West.

However, security forces in Nigeria are often ill-equipped and prone to bribe-taking, meaning law enforcement is weak. The country's northern border runs along the edge of the Sahara Desert — a vast, unguarded space where U.S. officials say Al Qaeda-linked militants operate.

The embassy warned U.S. citizens to be vigilant and aware of their surroundings at all times, saying "analysis has shown that individuals who appear to take basic personal security measures are often passed over by terrorist groups as a potential target."

With the sixth anniversary of the Sept. 11 attacks looming, analysts and security officials warn that terrorists could be gearing up. German authorities said an investigation into a failed plot there was aided by the suspects' increased activity ahead of the anniversary.