Al-Shabab has said it carried out a deadly bus attack on Kenya's coast that killed seven people, the latest in a series in the region that have left almost 100 dead.

In a statement on Saturday, the Somalia-based group said the attack near Witu in Lamu Country was "a clear response to the government of Kenya's false claim that they beefed up security in the area".

The group's military spokesman Abdulaziz Abu Musab told the AFP news agency that the group was "ready to act or attack anywhere necessary within Kenya".

Miiri Njenga, the police commissioner of Lamu County, said attackers hijacked the bus, which was travelling to Lamu from the tourist resort of Malindi, a 220km journey.

Daniel Kahosho, the head of Kenya Police Reservists in Mpeketoni, told the Reuters news agency the attackers blocked the bus with their car and then sprayed it with bullets.

"We still don't know how many of the passengers may have been hijacked. The bus is riddled with bullets." Kahosho said. Authorities believe many of the passengers in the 52-seater bus fled into a nearby forest.

"Two lorries which were carrying mangoes were also attacked..." Kahosho added.

About 100 people have been killed in attacks on Kenya's coast since mid-June. The al-Qaeda-linked group al-Shabab has claimed many of the attacks but the Kenyan government has suggested local politicians were also behind them.

In June the group said it carried out an attack in Mpeketoni town in Lamu county that killed at least 48 people.

The attacks near Lamu, an ancient trading port and tourist resort, have damaged Kenya's tourism industry and heightened volatility in a region where local communities have a history of conflict over land, water and other resources.