Investigations have been launched by police in Zanzibar following a series of bomb attacks targeting prominent personalities and a tourist hotel.
Zanzibar's stonetown is a popular tourist area
At the weekend, unknown assailants bombed the residence of a government minister and a government-appointed religious leader.
A BBC correspondent on the Indian Ocean island says two diplomats were present when a hand grenade was thrown at a tourist restaurant but did not go off.
No arrests have been made.
Diplomatic sources confirmed that a British and an American envoy were present at the restaurant during the failed bomb attack.
The BBC's Ally Saleh in Zanzibar says its difficult to tell who has been behind the attacks.
During the past two weeks, the government of Zanzibar has been locked in a confrontation with Muslim activists protesting at repressive laws.
However, the Muslim activists, have distanced themselves from the bomb attacks.
Last week, police in Zanzibar arrested five people whom they say were involved in violent activities which include the bombing of a bus owned by the Catholic Church in Zanzibar.