Fazul Abdullah Mohammed is believed to be in Kenya or Somalia
The United States has urged its citizens to postpone non-essential trips to Kenya, due to fears of possible terror attacks by groups linked to al-Qaeda.
The State Department fears that an attack could be timed to coincide with Thursday's celebration of the Prophet Muhammad's birthday, "Maulid".
The US statement follows a warning from the Kenyan Government that local al-Qaeda groups could be planning an attack this month.
Security Minister Chris Murungaru earlier released the photo of a man indicted for the 1998 bombings of the US embassies in Nairobi and Dar es Salaam.
The threat to aircraft by terrorists using shoulder-fired missiles continues in Kenya, including Nairobi
Fazul Abdullah Mohammed, from the Comoros Islands, was trying to get Kenyan documents and was operating between Kenya and Somalia, the minister said.
The FBI say he likes to wear baseball caps, dress casually and is good with computers.
The State Department has offered a reward of up to $25m for information leading to his arrest.
A statement said Kenyan security forces had been put on high alert and there would be a greater security presence around vital installations in the near future.
But the US warned "The government of Kenya might not be able to prevent such attacks".
Kenya's tourist industry was hit by the Paradise hotel attack
The department "recommends Americans defer all non-essential travel to Kenya at this time".
Following last November's failed attempt to shoot down a plane carrying Israeli tourists from a holiday resort near Mombasa, the State Department made a specific warning about air travel.
"The threat to aircraft by terrorists using shoulder-fired missiles continues in Kenya, including Nairobi," the department said.
"Seaports may also be targeted."
Fifteen people, mostly Kenyans, were killed in a suicide attack on the Israeli-owned Paradise Hotel near Mombasa at the same time as the attempt to shoot down the plane.
The US also warned of possible attacks across the region, mentioning countries from Djibouti to Zambia.
The UK and Australian Governments have also issued advice to their citizens this year warning that the al-Qaeda network or its sympathisers could attack countries in East Africa.