The minister of justice and constitutional affairs, Gen. Kahinda Otafiire has described Tuesday's terror attack in Kenya as a cowardly act and called upon Ugandans to be calm and vigilant.

The death toll from a bomb and gun attack by Islamist militants on an upmarket hotel complex in Nairobi has risen to 21. Another 28 people who were injured in the attack had been admitted to hospital

Otafiire assured Ugandans that security organs are on high alert to protect them from any threat.

"Ugandans have to be vigilant so that what happened to our neighbours Kenya does not happen here. We should work together to fight this terrorism," Otafiire said.

Kenyan security forces ended the attack early Wednesday after a 20-hour operation that rescued hundreds of people and left all five assailants dead.

President Uhuru Kenyatta had earlier announced the end of a 20-hour operation at the DusitD2 complex which saw hundreds of people rescued and all five jihadists "eliminated".

The bloody assault was claimed by the Al-Qaeda-linked Somali militant group Al-Shabaab, which said it was acting to avenge the decision by US President Donald Trump to declare Jerusalem as Israel's capital, according to the SITE monitoring group.

Chilling CCTV footage broadcast on local media showed four black-clad, heavily armed men calmly entering the luxury complex on Tuesday afternoon. A suicide bomb blast signalled the start of the attack.

Police chief Joseph Boinnet revised the death toll to 21 from 14, saying the victims included 16 Kenyans, one American, one Briton and three of African descent.

"Six other bodies were found at the scene and one police officer succumbed very suddenly to his injuries," he told reporters.

As police searched for further victims, bomb disposal experts were looking for any grenades left by the assailants.

As the first explosion and gunfire rang out in the leafy Westlands suburb on Tuesday, hundreds of terrified office workers barricaded themselves in the complex while others fled.

At least one suicide bomber blew himself up and others traded gunfire with security forces as the assault on DusitD2, a complex which includes a 101-room hotel, spa, restaurant and offices, unfolded.

Al-Shabaab has targeted Kenya since it sent its army into Somalia in October 2011 to fight the jihadist group.

The group also claimed responsibility for the July 2010 bombings in Kampala that claimed 74 lives.