* Somali militants al Shabaab claim responsibility
* Fleeing office workers say some colleagues still inside

At least one explosion and sustained gunfire at an upscale hotel and office complex in the Kenyan capital sent workers fleeing for their lives on Tuesday, an attack claimed by Somali militant group al Shabaab that it said was ongoing.

A plume of smoke rose above the 14 Riverside Drive complex. Firefighters extinguished three cars ablaze 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 are going on,’ said Abdiasis Abu Musab, the group's military operations spokesman.

The police were already treating the incident as a potential militant attack.

‘We have to go for the highest incident that could take place. The highest incident we have is a terror (attack),’ police spokesman Charles Owino told Citizen Television. Violent robberies are also common in Kenya.

‘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.

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