Foreign nationals have been warned that suicide bombers may be preparing attacks in the Kenyan capital Nairobi.
Armed personnel are on security alert in central Nairobi
Two hotels in the city centre have been identified as potential targets, while two other buildings have been evacuated because of bomb scares.
Armed Kenyan police have rushed into the area in response to a flurry of security alerts.
Al-Qaeda has struck in Kenya before, destroying the US embassy and attacking an Israeli-owned hotel and aircraft.
The first warning came from the American embassy which said there had been an anonymous tip about imminent terrorist attacks against the Hilton and Stanley Hotels, both popular with foreign tourists.
A spokesman said the information had not been corroborated and could be a hoax, but the United Nations also warned its employees not to visit the city centre.
The warning was followed by two bomb threats, the first by telephone to the KLM airline office where the US embassy had a small office until recently.
The second was made to an office block housing the French embassy and the British-owned Barclays Bank.
Both buildings have been evacuated.
The BBC's East Africa correspondent, Andrew Harding, says there is a jittery mood now in Nairobi, and the British High Commission has urged its citizens to be extremely vigilant.
In 1998, Osama bin Laden's al-Qaeda network destroyed the American embassy in Nairobi.
Last year, suicide bombers attacked a hotel in the coastal town of Mombasa and nearly brought down an Israeli charter plane.
More recently, Kenyan police say they uncovered a plot to bomb the new American embassy.
A number of Kenyans have been arrested, but our correspondent says that there is real concern that this country remains a likely terrorist target.
He says neighbouring Somalia has been identified as a key route to bring in arms and explosives.