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.

The pair were taken across the porous border into war-torn Somalia, where "they were moved from one Al-Shebab hideout to another in a bid to defeat any rescue efforts by Kenyan security forces," Kenyan police chief Joseph Boinnet said late Thursday.

He said the men, who were freed on June 25 but whose release was only made public on Thursday, were in "good health but traumatised."

Kenyan troops have been in southern Somalia since 2011 when they crossed into their neighbour to attack the Al-Qaeda-linked Shebab. They later joined the African Union force, AMISOM, which is supporting Somalia's internationally-backed government.

Boinnet gave no details of how they were freed, but thanked the "several security agencies which undertook this delicate rescue mission."

Under pressure in Somalia, Shebab is now increasingly targeting Kenya. It has also stepped up attacks during the Muslim fasting month of Ramadan, including killing 14 Kenyans on Tuesday in the northern town of Mandera.

In April, the Al-Qaeda-linked militants massacred 148 people at the region's Garissa University, most of them students.

In 2013, four Shebab gunmen killed at least 67 people in an assault on the Westgate mall in the capital Nairobi.