Kenyan wildlife rangers shot dead a stray male lion in the outskirts of Nairobi after it attacked and injured a local resident, the Kenya Wildlife Service said.

A mob had gathered around the lion, forcing the rangers to shoot it to avert further injuries, a spokesman for the agency said.

"It had injured somebody. There was a crowd that had formed around it, so it was practically impossible to capture it the way we planned to," Paul Udoto, communications manager for the wildlife service, told Reuters by phone.

The lion was the third in recent weeks to stray from Nairobi National Park on the outskirts of the capital. No injuries were reported in the first case on February 19, but one man was injured in the second, on March 18.

The wildlife service managed to capture the stray lions in the first two incidents and returned them to protected areas.

Images on social media showed the lion in the latest incident walking in a grassy area next to the fence of the national park around 9 am local time. They also showed residents gathering around the animal, some perched on the back of light trucks.

"The mob had formed and in the process somebody got injured, and by the time the veterinary and security teams got to the ground it was already beyond salvation," Udoto said. "With that commotion we risked more injuries or even possible deaths."

Nairobi National Park lies on the city limits, providing visitors views of lions, rhinos, giraffes, zebras and other wildlife against a backdrop of high-rise buildings.

Lions are occasionally spotted in the city close to the park after they find a way through fences that protect the built-up areas near the reserve.