Some of the Islamist attackers who stormed the Westgate mall in Kenyan capital Nairobi are suspected to be on the loose as a few of them are said to have escaped while the military-led operation was on.

The al-Shabaab assailants backed by al-Qaida laid a three-day siege on the Israeli-owned luxury shopping complex, leading to the death of at least 67 people.

Although the Kenyan authorities have claimed full control of the plaza following the onslaught, witness and police reports suggest the repercussions of the attack could be far from over.

An onlooker told the New York Times that one of the armed assailants quickly tore off his clothes and changed his outfit before merging into the crowd when the Kenyan forces were launching their anti-terror operation in the mall.

Kenyan security forces are suspecting two women, who were allegedly commanding the attackers in the shopping centre, had also managed to escape while the insurgents were battling the forces.

Many witnesses have been quoted saying that some of the hostage-takers were not Africans but foreigners.

Kenyan President Uhuru Kenyatta said five al-Shabaab militants were killed in the stand-off and 11 others involved in the siege have been arrested. It is still unclear how many of the detained suspects were gunmen who were inside the shopping complex, and how many were accomplices.

The attack, widely believed to be well-orchestrated and executed, has forced Kenyan officials to undertake serious investigation into the matter. It was also earlier reported that some of the militants could have rented a shop in the mall to gain complete access to the place.

Kenyan authorities have beefed up security in important places across the country.

Kenyan Foreign Minister Amina Mohamed had earlier confirmed al-Qaida's direct involvement in the operation and said the attack was part of a wider international terror campaign by extremist outfits.

Kenya has begun observing three days of national mourning for the victims of the attack.