NAIROBI — At least six people were killed when an Islamist suicide bomber and gunmen stormed an upmarket hotel and office complex in Nairobi, with police still trying to rescue people trapped inside the compound on Wednesday after a siege of over 12 hours.

Gunshots rang out sporadically in the capital as police tried to rescue survivors and flush out the attackers, however it was unclear how many were still hiding inside the complex.

After 12 hours trapped inside the complex, a group of dozens of people was freed at 3:30am (1230GMT), according to an AFP journalist on the scene, followed by fresh gunfire and a detonation.

The attack at the DusitD2 compound, which includes a 101-room hotel, spa, restaurant, and offices for local and international companies, began at 3:00pm on Tuesday with a massive explosion, heard five kilometres away at the AFP bureau.

The Al-Qaeda-linked Somali group Al-Shabaab, which carried out a notorious assault on a Nairobi shopping mall in 2013, claimed responsibility for the latest strike, according to the SITE Intelligence Group which monitors jihadist activities.

Kenyan police chief Joseph Boinnet said the attack began with an explosion targeting three cars in the parking lot and a suicide bombing in the foyer of the Dusit hotel.

He said "a number of guests suffered serious injuries" but did not give a figure for fatalities.

An AFP photographer saw the bodies of five dead, slumped over tables on a restaurant terrace in the complex. An official at the MP Shah hospital in Nairobi told Citizen TV seven wounded had been admitted, one of whom had died.

Meanwhile a police source, who asked not to be named, said he had seen as many as 14 dead.

Among the dead was an American citizen, a State Department official said.

‘Very confident’ attackers

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

"We have no idea what is happening. Gunshots are coming from multiple directions," said Simon Crump, an employee at an international firm who was hiding with his colleagues.

Crump was among a first wave of people evacuated from the office buildings surrounding the Dusit hotel after an hour of sustained gunfire as elite security forces engaged the gunmen.

A number of heavily-armed foreign forces, who appeared to be from embassies based in Nairobi, were at the scene alongside Kenyan security officers.

One survivor rescued from the building, speaking to a local television station, said the attackers were "very confident; they were people who knew what they were doing".

CCTV footage broadcast on local television stations showed four attackers, clad all in black and heavily armed, entering a courtyard in the compound at the start of the attack.

‘A flash and a bang’

John Maingi said there had been "a flash of lights and a loud bang" at the Secret Garden restaurant where he works.

"When I peeped outside I saw a human leg which has been cut off. We hid in the room and then some police officers rescued us," he said.

Reuben Kimani, a barista who was rescued after several hours trapped inside the hotel, said he had recognised one of the attackers as a client he had served coffee to in recent days.

"I knew one of them because he had a big scar on one of his hands," he said. "I saw them. They shot six of my friends, four didn’t die but two succumbed."

Several Nairobi establishments popular with westerners, such as the J’s and Alchemist bars and the Village Market shopping centre, announced on social media that they had closed for security reasons.

Shabaab

The attack at DusitD2 is the first in Nairobi since gunmen stormed the city’s Westgate shopping mall in 2013, killing at least 67 people. The attack and ensuing siege lasted about four days.

That assault was also claimed by Somalia’s Shabaab, fighting since 2007 to overthrow the internationally-backed government in Mogadishu.

The Westgate attack resulted in many upscale establishments and shopping centres in the capital - including the Dusit - putting up strict security barriers to check cars and pedestrians.

The Shabaab targeted Kenya after it sent its army into Somalia in October 2011 to fight the jihadist group.

On April 2, 2015, another Shabaab attack killed 148 people at the university in Garissa, eastern Kenya.

In its statement, the group noted the attack came exactly three years after its fighters overran a Kenyan military base in Somalia.

"This attack on Nairobi hotel came as Kenyans and their media are commemorating (the) El Adde attack," it said.

The Shabaab claimed it killed more than 200 soldiers in that assault. The government has refused to give a toll or disclose details. — AFP