A series of explosions at a bus station in the Kenyan capital Nairobi have killed at least four people and wounded 40 others, the Kenyan Red Cross has said. 

The Red Cross reported on its official Twitter account that eight out of the 40 injured people admitted to hospital on Saturday are in a critical condition.

Witnesses reported that people in a moving car hurled three grenades at the terminal in Nairobi, Charles Owinom the police spokesman, said. Witnesses told the Reuters news agency they believed there had been up to four blasts, but there was no official confirmation of multiple explosions.

Dan Mutinda, a disaster risk reduction officer with Kenya's Red Cross told Al Jazeera that doctors were seeing to the victims of the blast, but that many also suffered bullet wounds.

"It appears that it was not just a series of blasts, people were also sprayed with bullets.

"The Kenyan vice president visited some of the injured, here at the hospital, and this shows the magnitude of the incident," he said.

Al-Shabab

Kenyan police immediately blamed al-Shabab, the Somali Islamist armed group, that Kenyan troops are currently fighting in neighbouring Somalia.

"This is a cowardly act by al-Shabab elements," Charles Owino, police spokesman told reporters at the bus station.

"But we will not relent in the war. We will get them and we will continue with the war."

The blasts are the latest in a string of small arms, bomb and grenade attacks that have killed scores of people since Kenya sent troops across the border into neighbouring Somalia in October.

During the October attacks, grenades were thrown into a bar and another bus station, both not far from the scene of Saturday's attack, killing one person and wounding more than 20.

A Kenyan man arrested shortly afterwards pleaded guilty to the October bus station attack and being a member of al-Shabab.

He was sentenced to life in prison.