Somali Islamist group al Shabaab has claimed the attack at an upscale hotel and office complex in the Kenyan capital, adding it was still ongoing.

At least one person is dead and four injured, reports say, as an explosion and sustained gunfire was heard, sending workers fleeing for their lives.

It was not clear how many attackers took part, The Associated Press reported.

Gunfire continued several minutes after the first reports as ambulances, security forces and firefighters rushed to the scene.

The complex in Nairobi’s Westlands neighbourhood includes a large hotel known as DusitD2, along with banks and offices.

Black smoke rose from the complex. A bomb disposal unit arrived, and vehicles were cordoned off for fear they contained explosives.

Police said they detonated a car that had explosives inside. An unexploded grenade was also seen in a hallway at the complex, AP said.

People were rushed, some carried, from the scene Some ducked behind cars, screaming. Others appeared to take cover behind fountains and other features in the lush outdoor complex.

Firefighters extinguished a blaze involving three cars by the entrance as armed security personnel headed in and other armed officers escorted shocked workers out, many with their hands up.

A woman shot in the leg was carried out and three men emerged covered in blood. Some office workers climbed out of windows. Many told Reuters that they had had to leave colleagues behind, still huddled under their desks.

“There’s a grenade in the bathroom,” an officer yelled as police rushed out from one building.

A picture of the grounds on Twitter showed what appeared to be a human leg lying on the ground.

“We heard a loud bang from something that was thrown inside. Then I saw shattered glass,” Geoffrey Otieno, who works at a beauty salon in the complex, told Reuters. “We hid until we were rescued.”

Kenya has often been targeted by al Shabaab, who killed dozens of people in a shopping centre in 2013 and nearly 150 students at a university in 2015.

“We are behind the attack in Nairobi. The operations is going on,” said Abdiasis Abu Musab, the group’s military operations spokesman.

“I just started hearing gunshots, and then started seeing people running away raising their hands up and some were entering the bank to hide for their lives,” a woman working in a bank in the complex said, adding she had heard two explosions.

“We are under attack,” another person in an office told Reuters, then hung up.