The group of fighters attacked Arabiya, a trading centre 60 kilometres from Mandera, and engaged police in a firefight before overpowering them and taking all the guns and bullets from the local police post.
"Arabiya was attacked. We believe it's al Shabaab. They destroyed, burnt a communication booster and took ammunition at the police post," North Eastern Provincial police commander Leo Nyongesa told Reuters by phone.
There were no injuries or deaths reported.
Kenya ordered its soldiers across the border in October to crush the al Qaeda-linked al Shabaab who it said had attacked its security forces and tourists inside Kenya.
The latest incident comes days after grenade attacks in the frontier town of Garissa killed six, and a roadside bomb killed a soldier in Mandera town.
Nyongesa said police had arrested five people suspected to be involved in the Garissa attack.
Although there appears to be little progress on the ground in Somalia as torrential rains bog down operations, more airstrikes have been launched on al Shabaab strongholds in recent days and there have been skirmishes and bomb attacks in northern Kenya.
"The week has been very intense with air operations that have been aimed at decimating and degrading al Shabaab capacity to be able to plan and launch operations in the country," Kenya Defence Force's Colonel Cyrus Oguna told a news conference in Nairobi.
During the operation, now in its sixth week, four soldiers have died in direct combat, in addition to five killed in a helicopter crash at the start of the incursion. One was missing at sea.
Oguna said nine people arrested Friday in separate incidents on Lamu island were suspected of being al Shabaab members.
Kenya is the latest foreign power to try to stabilise Somalia, which has been mired in violence for two decades since the overthrow of dictator Mohamed Siad Barre in 1991 allowed first warlords, then Islamist militants, to step into a power vacuum.
Friday, officials from Ethiopia, whose soldiers have been in Somalia before, said the country will deploy troops to the anarchic Horn of Africa state for a "brief period" to help Somali and Kenyan forces battling al Shabaab.
In an emailed statement responding to Ethiopia's plan, al Shabaab said: "The people of Somalia shall never accept or live under the humiliation of occupation and the spirit of resistance shall not fade as long as a single invader remains alive on Somali soil."
(Additional reporting and writing by George Obulutsa; Editing by Sophie Hares)