Five Kenyan policemen escorting a passenger bus have been killed after suspected al-Shabab fighters ambushed their convoy in the northeast of the country, according to officials.

The officers came under attack early on Monday near the remote Kenyan town of Elwak, on the border with Somalia.

The bus driver sped away, but the police vehicle was set on fire after being hit by a suspected rocket-propelled grenade.

Two of the victims were burned beyond recognition, Job Boronjo, the police commander in Mandera county, told the Associated Press news agency, adding that four officers survived.

"This was a well-arranged ambush by the militants," he said, noting that the attackers were believed to had fled towards the Somalia border.

Ali Roba, the governor of Mandera, confirmed that five policemen were killed in "the attack by al-Shabab".

There was no immediate statement from al-Shabab, the armed group fighting to overthrow the internationally backed government in Mogadishu, which is protected by 22,000 African Union troops, including Kenyan soldiers.

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Al-Shabab fighters have made a series of deadly incursions into neighbouring Kenya, including the 2013 killing of at least 67 people at the Westgate Mall in the capital, Nairobi, and the massacre of 148 people at a university in the town of Garissa in April 2015.

The armed group has also targeted the Mandera region in the past, including in late December 2015 when a Muslim teacher was shot after refusing to be separated from Christian passengers during another attack on a bus travelling to Nairobi.

On patrol with security forces in Somalia

Source: Al Jazeera and agencies