A spokesman for Somalia's Shebab rebels claimed that the Al-Qaeda-linked group's fighters had carried out another attack in the area.
The Red Cross said nine people died and one person was missing in the locality of Gamba, while four people were killed in Hindi, a trading post near Lamu island. The areas were attacked late Saturday, authorities said.
"We had attacks at night where people were killed and houses destroyed. We have mobilised our officers and we are on the ground. We are calling on the public to work closely with us," said Robert Kitur, a senior Lamu police official.
Police said unidentified gunmen also torched several houses and attacked Gamba's police station, freeing a suspect held over last month's attacks. One policeman was among the dead, officials said.
In a statement issued just hours after the violence, Somalia's Shebab rebels said they were responsible.
"The attackers came back home safely to their base," Shebab military spokesman Abdulaziz Abu Musab said, saying the militants had killed 10 people.
The Shebab also claimed responsibility for last month's attack at Mpeketoni, saying it was in retaliation for Kenya's military presence in Somalia as part of the African Union force backing the country's fragile and internationally-backed government.
Survivors of the massacre in Mpeketoni and a similar attack the following night in a nearby village reported how gunmen speaking Somali and carrying Shebab flags killed non-Muslims and said their actions were revenge for Kenya's presence in Somalia.
Kenyan President Uhuru Kenyatta, however, denied that the Shebab were involved and instead blamed "local political networks" and criminal gangs, and said that the victims had been singled out because of their ethnicity.
The attackers appeared to have targeted Mpeketoni because the town is a mainly Christian settlement in the Muslim-majority coastal region, having been settled decades ago by the Kikuyu people from central Kenya, the same tribe as Kenyatta.
Police also arrested alleged separatists from the Mombasa Republican Council, a group that campaigns for independence of the coastal region, as well as the governor of Lamu county, who is an opposition politician.
The unrest in the coastal region has badly dented Kenya's tourist industry -- a key foreign currency earner and massive employer for the country -- at one of its traditionally busiest times of the year.
Lamu island is a well-known tourist destination whose ancient architecture is listed as a UNESCO World Heritage site.
