Gunmen on a motorcycle have killed at least four people and injured several others in the Kenyan city of Mombasa.

Police were quoted by local media as saying that the gunmen had fired indiscriminately at passers-by.

The port city has seen a wave of violence in recent months, with a number of bombings and gun attacks.

The violence has largely been attributed to Somali al-Shabab militants but many say local political rifts are to blame.

Witnesses said the gunmen rampaged through the streets of Kenya's second-largest city.

'Shooting carelessly'

Peter Musyoki, a resident in Mombasa's Likoni area who saw the shooting, said the two masked men were armed with a rifle and a pistol.

"I saw two men dressed in black with a red ribbon around their heads," he said. "They walked on foot and were just shooting carelessly at anyone they saw."

The Reuters news agency say the gunmen also handed out leaflets saying the attack was retribution for last month's violence in Mpeketoni, a town about 300km (185 miles) north of Mombasa.

More than 60 people were killed in two days of violence there in June and President Uhuru Kenyatta blamed "local political networks" for the attacks.

Ethnic tension

Most of the dead were ethnic Kikuyus, like the president.

Critics said Mr Kenyatta was trying to put the blame on his rival Raila Odinga, an ethnic Luo, whom he defeated in last year's presidential election.

The leaflets distributed by the gunmen on Sunday said the attack was "revenge for our brothers who were killed in Mpeketoni".

It continued: "You Luos, you wont stay in peace, and you Raila [Odinga], if you have anything to do, just do, we are not fearing you at all."

Correspondents say political allegiances in Kenya tend to follow ethnic lines and inter-ethnic tensions have led to bloody violence in the past.

A disputed poll in 2007 sparked weeks of ethnic violence that left more than 1,200 people dead and badly damaged the economy.

Kenya - and Mombasa in particular - was once a popular tourist destination but it has suffered from a wave of attacks in recent years.

Several foreign governments, including the US and Britain, have issued travel warnings advising their citizens to avoid Mombasa.

As well as political tension, Somalia's Islamist militia al-Shabab say they have been behind several of the recent attacks in Kenya.

On Friday, they said they were responsible for an attack near the town of Witu, some 50km (30 miles) from the resort island of Lamu, in which seven people were killed, including four police officers.