The United Nations refugee agency UNHCR has expressed concern over the recent mass arrest of Somali refugees in the Kenyan capital, Nairobi.

The UN agency said Tuesday that the detained Somali asylum seekers and refugees should be protected against arbitrary detentions.

According to the country’s Interior Minister Joseph Ole Lenku, more than 4,000 people have been arrested in recent days.

Among those detained included more than 1,000 Somalis who are reportedly being held by Kenyan security forces in a soccer stadium in the capital.

There have been reports that the detainees are lacking food, water and medicine in the stadium.

The UN and international organizations have been denied access to the meet the detainees.

The Kenyan police said the mass arrests were a part of ‘a security operation to fight terrorists’ and followed a recent deadly attack in Nairobi.

On March 31, at least six people were killed and several others injured in a grenade explosion near a food kiosk and bus stop in Nairobi.

The blast took place in the capital’s Eastleigh -- a suburban area which is often known as 'Little Mogadishu' because of its predominantly Somali population.

Following the attack, Kenyan security forces launched a full-scale operation in the suburb, mainly targeting Somali nationals living in Nairobi.

Somali community leaders have criticized security forces for forcing their way into homes and arresting people. Local people have also accused security forces of being corrupt, brutal and using blanket punishment.

No group claimed responsibility for the Monday attack. Such explosions are, however, usually blamed on Somalia's al-Shabab group. The group had claimed responsibility for an attack on a Nairobi shopping mall back in September 2013 which left at least 67 people dead.

Al-Shabab had said the last September raid was in retaliation for Kenya’s military presence in Somalia.

The Kenyan forces are part of the African Union Mission in Somalia (AMISOM) fighting against the al-Shabab group.

Somali has been the scene of clashes between government forces and al-Shabab since 1991.