Kenyan police chief Joseph Boinnet confirmed "there was an incident where a police lorry hit an improvised explosive device" yesterday, but gave no casualty toll.
One senior police officer, who asked not to be named, said five officers were killed in the blast as the truck drove towards Mpeketoni, in the coastal Lamu region.
Kenya's Daily Nation newspaper also reported five police were killed, while The Standard reported six, and The Star said seven died.
A Shebab statement said more than 100 Kenyan soldiers were killed and others captured. Kenya has so far refused to say how many of its soldiers were killed, injured or remain missing.
More From This Section
Kenyan President Uhuru Kenyatta said in a statement that the Shebab would "have no time to breathe" and vowed revenge.
He is due to attend a memorial service on Wednesday, alongside visiting Nigerian President Muhammadu Buhari.
The army however said it was not leaving Somalia and remained committed to the fight.
"There is a reason that took us to Somalia, which is to liberate and pacify those areas, and the mission is still on," army spokesman David Obonyo said.
Kenyan soldiers vacated at least two military bases in El-Adde and Badhaadhe, witnesses said, adding that Shebab fighters occupied the bases after the withdrawal.
"Shebab fighters took control of El-Adde after the Kenyan soldiers pulled out," said Abdulahi Mohamud, a traditional elder.