Gunmen blasted their way into a hotel and office complex in the Kenyan capital of Nairobi on Tuesday, killing at least 15 people, Reuters reported.

The attack was claimed by the Somali Al-Shabaab terrorist organization.

Nearly 11 hours after the assault began at Nairobi's upscale 14 Riverside Drive complex, bursts of gunfire and an explosion were heard in the area, suggesting the situation was not yet under control despite government assurances.

Scores of people were still hiding inside the complex, a security source said, and a first responder in touch with many of them said some needed first aid for gunshot wounds. By 1:00 a.m. local time, 15 bodies had arrived at Chiromo mortuary and more were expected, an attendant told Reuters.

Identification papers indicated that 11 were Kenyan, one was American and one was British, he said. The other two did not have documents on them.

Interior Minister Fred Matiang'i had said at 11:00 p.m. that all buildings had been secured and scores of people evacuated from the scene. He did not comment on the attackers' whereabouts and said security forces were still "mopping up".

Nairobi was hit in 2013 by a terrorist attack which targeted the Westgate Mall and in which 67 people were killed.

That attack was also claimed by Al-Shabaab as retaliation for Kenya's military presence in Somalia.

American airstrikes in Somalia have several times targeted senior members of Al-Shabaab since the Nairobi attack.