Kenyan police have released a British man without charge after he was arrested in Nairobi following the attack on a shopping mall by Islamist militants, British sources said on Monday.

"We can confirm that a British national has been released from custody in Nairobi," a spokeswoman for the Foreign Office said.

Britain on Saturday said a sixth British citizen had been confirmed dead in the four-day siege at a Nairobi shopping mall by Islamist gunmen.

"A sixth British national has been identified amongst those killed in the tragedy in Nairobi. We are providing consular assistance," a Foreign Office spokesman said.

Britain on Thursday revised down the number of British people killed in the Nairobi shopping center attack from six to five, saying one of those previously identified was Kenyan.

"We can now confirm that on present information five British nationals have been killed in the recent terror attacks in Nairobi," a spokesman for the Foreign Office said.

Fred Bosire, who works in the meat section of the supermarket at Nairobi's Westgate shopping mall, was having an ordinary, busy Saturday before the carnage began.

A staffer at the Nakumatt supermarket located in the back of the complex, he was immediately trapped along with hundreds of colleagues and shoppers when gunmen marched in and raked the supermarket aisles with automatic weapons fire.

The Somalia-based militants who stormed a Kenyan mall had a detailed plan and had hidden weapons at the scene beforehand, according to U.S. officials cited by the New York Times Wednesday.

According to the report, a hand-picked group of English-speaking fighters from the Shebab, an Islamist rebel group, had trained for the assault in Somalia for weeks beforehand.

A Frenchman caught up in the Nairobi mall attack who survived by hiding for five hours in his bank said Wednesday he had an overpowering desire to kill the militants when he saw a child's corpse.

The man, who identified himself only by his first name, Eric, said that his initial feelings of helplessness turned to rage when he saw the body and thought it could have been his daughter.

A British national has been arrested in Nairobi following the bloody attack on a shopping mall by armed Islamists, a spokeswoman for the Foreign Office said on Wednesday.

"We can confirm the detention of a British national in Nairobi and we are making contact to offer standard consular assistance," she told AFP.

Kenyan troops and rescue workers scoured the wreckage of a Nairobi shopping mall Wednesday for bodies and booby-trapped explosives after a four-day siege by Islamist gunmen left 67 dead and dozens more missing.

Rescuers wore face masks and some soldiers wrapped scarves around their mouths because of an overpowering stench inside the Westgate center, once the capital's most upmarket mall. A large part of the complex has collapsed after heavy explosions and a fierce fire.

One of the Islamist attackers besieging a Nairobi shopping mall handed chocolate to a four-year-old British boy caught up in the crisis and asked for forgiveness, his uncle told a newspaper on Tuesday.

Four-year-old Elliott Prior, who had been shopping with his mother and sister at the Westgate mall when it came under attack on Saturday, confronted one of the militants, telling him "you're a very bad man," his uncle told The Sun.

The United States is monitoring the bloody siege in a Kenyan shopping mall and will stand with the country's people in the days ahead, U.S. Secretary of State John Kerry said Monday.

"Ruthless and valueless terrorists remain a serious challenge everywhere in the world as we all know, whether it's in downtown Manhattan or in a mall in Nairobi, or anywhere else in the world," Kerry said in remarks to a conference at the United Nations.