At least 36 people have been killed in an attack on a stone quarry in Mandera, northern Kenya, local sources told Al Jazeera.

About 20 gunmen, described by the Kenyan government as "heavily armed bandits" opened fire at the quarry in the early hours of Tuesday morning after separating Muslims and non-Muslims, sources said.

Several people were beheaded, while others were shot in the back of the head, sources told Al Jazeera.

Al-Shabab claimed responsibility for the attack, calling it "another successful operation carried out by the Mujahideen".

"This latest attack was part of a series of attacks planned and executed by the Mujahideen to serve as a response to Kenya's occupation of Muslim lands and their ongoing atrocities therein, such as the recent air strikes on Muslims in Somalia," al-Shabab spokesman Sheikh Ali Mohamud Rage said in a statement emailed to journalists.

The dead are reportedly miners who worked in the quarry, located just outside the town. Shortly before the attack they were woken by the gunmen and forced to leave the tents where they were sleeping.

Increased security

Al Jazeera's Catherine Soi, reporting from Nairobi, said that the Red Cross was on the scene and that the military and army were heading to the site, which is just 5km from the border with Somalia.

"We are being told that many of the people who work in the quarry are not from Mandera," she said, adding that those killed were non-Muslims.

"This comes at a time when security in Mandera has been beefed up because of recent attacks. We just returned from Mandera and the security forces there assured us they are in control of the situation, but they said that the border there is also very porous."

The scene of the latest attack is about 30km from the November 22 raid on a bus in which 28 non-Muslims were separated from the Muslim travellers before being executed.

The shooting in the quarry comes after an attack on a night club in the Kenyan town of Wajir on Monday night.

One person was killed and at least 12 injured as gunmen hurled grenades and sprayed bullets at the building, the government said.

The Kenyan government, in a statement, said investigations were under way to look into both incidents.