The United States embassy in Kenya has warned of an "imminent" threat of an attack on the port of Mombasa.
US officials have been told not to travel there until 1 July.
There has been a series of grenade attacks in Mombasa and the capital Nairobi since Kenya sent troops into Somalia to battle al-Shabab fighters.
Kenyan accuses the group of being behind various kidnappings on Kenyan soil and destabilising the border region.
Al-Shabab has repeatedly threatened to stage revenge attacks in Kenya.