The Kenyan Red Cross said five civilians and two police officers died late yesterday when armed men opened fire first on a bus, then on a car and a police vehicle, near the town of Witu, 50 kilometres from Lamu island.
Local authorities said four of the seven dead were police officers.
Today, police announced a month-long night curfew in the region, where repeat attacks have left dozens dead and driven tourists from what was once a renowned island paradise.
"The attacks are a clear response to the government of Kenya's false claim that they beefed up security in the area," said Shebab military spokesman Abdulaziz Abu Musab.
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He added that the withdrawal of Kenyan troops from Somalia "is the only hope for a sort of normality".
But the group denied involvement in a separate incident -- an alleged suicide bomb attack on a militia leader in Somalia that reportedly killed six people today.
According to police accounts of the first attack, gunmen opened fire on a bus travelling from Mombasa to Lamu. They then targeted a police car which stopped to intervene, and also hit a third car.
Somalia's Shebab has carried out similar attacks in the past, saying they were in retaliation for Kenya's military presence in Somalia as part of the African Union force supporting the country's fragile government.
Lamu, a UNESCO World Heritage site, has been hit by a series of attacks since mid-June which has left a total of 94 dead, according to a Red Cross tally.
The attacks have fuelled divisions on the coast, a region where radical Islam, ethnic tensions and land disputes are an explosive cocktail.