Some were hacked to death with knives, others shot and executed at close quarters, mirroring tactics used in earlier attacks claimed by Somalia's Al-Qaeda-linked Shebab.
The attack took place overnight on a small village near the town of Witu, on the mainland some 50 kilometres west of the tourist island of Lamu.
Lamu County Commissioner Stephen Ikua called it an "unfortunate attack", the third in the area this month.
Five bodies were initially found, Ikua said, but a police source later said that six more bodies were found, taking the toll to 11.
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Several people were reported to have been wounded in the attack and taken to nearby hospitals for treatment, local councillor Athaman Badi said.
There was no immediate claims of responsiblity.
"We have sent our officers to the ground to get more details... They are on the ground pursuing the attackers," Ikua added.
The village is reported to be a mix of Kenyan ethnic groups.
Attacks last week on the nearby coastal Mpeketoni district left at least 60 dead.
"The style of the attack is very similar to that at Mpeketoni, it seems like it is the same men," Maisori said.
Last week's attacks were claimed by Somalia's Islamist Shebab insurgents, though President Uhuru Kenyatta blamed "well-planned, orchestrated and politically motivated ethnic violence" carried out by "local political networks".
Kenyatta's accusations have stoked already tense political rivalry between the government and opposition parties, and raised fears of renewed ethnic tension.