Nairobi, Kenya (CNN) An evacuation of a hotel complex in the Kenyan capital Nairobi is still ongoing, hours after suspected Islamist extremists launched an attack killing at least 11 people, including a US citizen.

Tuesday's coordinated assault by armed gunmen on the Dusit D2 compound, an upmarket cluster of shops and hotel facilities, represents the most high-profile terror attack in the East African country in several years.

Authorities reported that "scores" of Kenyans and foreign nationals had been evacuated from the compound, and that the site had been secured, but sporadic gunfire continued to be heard hours after the all-clear had been given.

A source at the hotel assisting with security called the death toll "conservative" and expected it to rise. "I saw six dead on the footpath exit over the river and five more at the secret garden cafe," said the source. The US state department confirmed a US citizen was killed in the attack, but provided no further details.

A woman is escorted from the Riverside Drive complex in Nairobi.

Responsibility for the assault, described by police as a "suspected terror attack," has been claimed by Somali Islamist militant group Al-Shabaab, according to the SITE Intelligence Group.

Read More