UPDATE: Al-Shabaab, the Islamist militant group based in Somalia, have claimed responsibility for the attack.
At least five people have reportedly been killed in a suspected terrorist attack after gunmen ambushed an upscale shopping mall in the Kenyan capital, Nairobi, on Saturday and opened fire "indiscriminately."
British nationals have been told to avoid the upmarket area as armed police exchange fire with gunmen at the Westgate centre in the Westlands district - one of the city's most exclusive shopping malls – in an incident described by police chief Benson Kibue as a terrorist attack.
The Foreign Office said it is "urgently looking into" the incident.
Up to 10 terrorists involved in the attack exchanged gunfire with police, he told the AP news agency.
One witness said the attackers threw grenades into the building at about midday local time in an incident that appeared to be an attack rather than an armed robbery.
There are fears that there may be some hostages in the mall, often frequented by wealthy Kenyans and expatriates, the NTV channel reports.
Abdi Osman Adan, a journalist in Nairobi, told Sky News staff at a supermarket and a jewellery store had reportedly been taken hostage.
According to the centre's website, it is the city's "premier shopping mall" and offers a "serene and safe enviornment away from the city centre hubbub".
Armed police shouted "get out, get out", and scores of shoppers fled the building, Reuters reported.
"They don't seem like thugs, this is not a robbery incident," said Yukeh Mannasseh who was on the mall's top floor when the shooting started.
"It seems like an attack. The guards who saw them said they were shooting indiscriminately," Reuters reported.
Nairobi police chief Benson Kibue told the Associated Press the gunmen had besieged the building, and officers were on the ground engaging the attackers. He said the men were trying to rob a shop within the mall.
Some of those running from the mall said the attack may have been a bank robbery gone bad.
Armed police took cover behind vehicles outside. "We have officers at the scene trying to get out the group shooting inside," a police official told the AFP news agency.
"Officers are approaching the situation with caution because there are innocent civilians inside," he said.