He said 11 other suspects had been arrested; authorities had previously announced the arrest of seven at the airport and three elsewhere.
"These cowards will meet justice as well their accomplices and patrons, wherever they are," Kenyatta said.
Kenyatta declared three days of national mourning starting Wednesday.
At the Westgate mall, there were no immediate signs of the Kenyan Security forces closing their operation.
Two Kenyan soldiers who had recently been inside the mall told The Associated Press shortly before the president spoke that the operation was effectively finished, but they said security forces were still combing the facility and had not definitively cleared all the rooms inside. They spoke on condition of anonymity because they were under orders not to speak to the media.
Kenyan forces had for two days said they were in the "final phase" of the operation, only to be battled back by the militants inside the building.
Explosions rang from the upscale Westgate mall in Nairobi throughout Tuesday, and the chatter of gunfire from inside the building could also be heard. Fresh smoke rose from the building in the afternoon.
The Kenyan Red Cross had previously said 62 people had been killed, and it seemed certain that the number of confirmed deaths would rise as security forces search the building.
Nairobi's city morgue had already braced for the arrival of a large number of bodies of people killed, an official said.
Kenyan Red Cross spokesman Abbas Gullet said it was still not known how many more may be dead inside the building.
"It is certainly known that there are more casualties," he said.
A government official told The Associated Press that the morgue was preparing for up to an additional 60 bodies, though the official didn't know an exact count. The government official insisted on anonymity so he would not face retribution from government officials.
Earlier Tuesday the al-Qaida-linked attackers used social media to give accounts of the fighting inside the mall that conflicted with the government reports.
"There are countless number of dead bodies still scattered inside the mall, and the mujahideen are still holding their ground," the Somali rebel group al-Shabab said in one Twitter message considered to be genuine.