"We have four people dead from the incident that occurred between Dadaab and Doble," said the regional commissioner Mohamud Saleh.
Three of those killed were employed by an NGO called Adeso, which was founded by a Somali activist and receives funding from the US and European Union for its work in the Horn of Africa, according to its website.
The incident occurred near the town of Dadaab -- home to one of the world's largest refugee camps.
Inspector General Joseph Boinnet of the Kenyan police warned that more such attacks were likely during the holy month of Ramadan.
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Since 2007, the Al-Qaeda linked Shabaab has fought to overthrow successive internationally-backed governments in Somalia but began attacking Kenya in 2011 after Nairobi ordered its troops into Somalia to fight the militants.
Kenyan soldiers are now part of a 22,000-member African Union mission fighting there.
In 2013, Shabaab gunmen raided a shopping mall in the capital Nairobi killing 67 people, and in 2015 a similar attack on a university in Garissa left 148 dead.