Two Kenyan policemen have been freed after over two years being held hostage in Somalia by Shebab militants, Kenya's police chief said.

The police constables, Joseph Wambugu and Fredrick Chirchir, were kidnapped in an attack in May 2013 in Kenya's northeastern Garissa district, when four other officers were killed.

Shebab militants killed at least 14 workers in northern Kenya on Tuesday, officials said, the latest in a wave of attacks in the east African nation by the Somali-led Islamists.

The attack occurred just outside the town of Mandera in Kenya's far northeastern region, which borders Ethiopia and war-torn Somalia, and comes a little more than two weeks before U.S. President Barack Obama is due to make his first presidential visit to Kenya, where his father was born.