Three people have been killed and several wounded in two explosions in the Kenyan coastal city of Mombasa, police say.

Three people died in the busy Mwembe Tayari area of the city centre after a grenade was reportedly thrown into a bus that had just arrived from Nairobi.

In a separate incident at the luxury seaside Reef Hotel, suspects threw a bag with an explosive device, guards at the hotel said.

A roof of one building was ripped off by the blast and part of its wall collapsed.

The hotel's management said all its staff and guests were safe.

There was no immediate claim of responsibility, although the city has been on high alert for possible attacks by Somalia's Al Shabaab rebels or local Islamist militants.

The Al Qaeda-linked group has carried out several attacks in Kenya since 2011, when Kenya sent troops into Somalia to fight Al Shabaab rebels.

Al Shabaab claimed responsibility for the high-profile attack on Nairobi's Westgate shopping mall last year in which at least 67 people were killed.

Mombasa, a Muslim-majority port city that is one of the main gateways to east Africa as well as a popular tourist destination, has been hit by sporadic unrest in recent months.

In March, two people were arrested along with a car expertly packed with explosives.

Intelligence sources say they believe the car was rigged in Somalia and driven into Kenya for a high-profile bombing.

Also in March, six worshippers were shot dead in a church in Likoni near Mombasa.

The following week a local firebrand Islamist cleric was gunned down in the city, the third prominent hardline cleric to be killed in or around the city in as many years.

ABC/wires