One person has been killed and several others wounded in a hand grenade attack on a bar in the Kenyan tourist hub of Mombasa, police say.

Sunday's attack comes a day after the US embassy in Kenya issued a warning on the threat of an imminent attack in Mombasa, and Kenyan police arrested two Iranians on suspicion of planning bomb attacks.

"The attack occurred at about 10pm. One person has died and we have several others injured," a police officer in Mombasa said.

The US embassy statement said: "This is to alert all US citizens in Kenya, or planning to travel to Kenya in the near future, that the US embassy in Nairobi has received information of an imminent threat of a terrorist attack in Mombasa.

"All US government travel to Mombasa is suspended until July 1."

France's embassy in Nairobi also urged its citizens to be "extremely vigilant" in Mombasa and the surrounding area.

The warnings came as Kenyan police said they had detained two Iranian nationals over suspected links to a terror network planning bombings in Mombasa and in the capital, Nairobi.

One of the Iranians was detained on Wednesday in Nairobi, the other on Thursday in Mombasa, police said.

Police in Nairobi have also seized bomb ingredients from two young men who were stopped during a routine patrol, according to a police officer who spoke on condition of anonymity on Saturday.

The call to shun Mombasa, which lies on the Indian Ocean, is likely to deal a further blow to tourism, a key revenue sector for the East African nation that only recently recovered from the violent fallout of a disputed 2007 presidential election.

Since Kenya invaded southern Somalia in October 2011 to help oust al-Qaeda-linked Shebab fighters, it has seen a wave of grenade attacks and kidnappings of foreign tourists blamed on the Shebab or their supporters.