The radical Somali-based Islamic militant group Al Shabab took a "massive number of victims" at an upscale shopping mall in Nairobi, according to the Kenyan Red Cross, while the building remains under siege by Kenya's security forces.

The president of the African nation, Uhuru Kenyatta, said that for the moment 39 deaths have been confirmed and 150 people were wounded, though the number is expected to increase because Westgate mall on a Saturday morning would normally have several hundred shoppers on the premises.

The radical Somali-based Islamic militant group Al Shabab, which in February announced its formal union with al-Qaeda, claimed in a Twitter post that it was responsible for the attack on the Westgate mall in Nairobi, saying gunmen had killed "more than 100" people inside the shopping center thus far in retaliation for the Kenyan military's participation in the U.N. mission in Somalia.

Kenya's armed forces are backing the Somali government against Islamist militias such as Al Shabab.

The militant group added on Twitter that the "battle is continuing" inside the mall.

The attack apparently began Saturday morning when some 10 armed men speaking in Arabic entered the packed mall and began firing at anyone unable to respond in their language.

Red Cross workers told Efe, for their part, that some of those killed had been shot in the head and one person had been decapitated.

Army troops were dispatched to the shopping center to subdue the attackers and free the employees and customers who were still inside.

At the time the attack erupted, numerous children were believed to be inside the mall taking part in a youth cooking workshop.

Kenya's Interior Ministry has issued repeated calls on Twitter for people to stay away from the mall for their own safety.

Westgate mall is located in one of the most upscale areas of Nairobi and is frequented by that city's Western expatriate community. EFE