PARIS (AFP) - For Somalia's Shebab, the authors of a deadly attack on a posh Nairobi shopping mall, every hour spent at the top of the world's news agenda is priceless publicity, experts say.

The Al-Qaeda-linked insurgents have claimed the attack, which began midday on Saturday, sending gunmen into the complex with grenades and automatic weapons and rocking the world with terrifying and protracted scenes reminiscent of the 2008 attacks on Mumbai.

The Shebab group said it wanted to avenge Kenya's military intervention in Somalia where African Union troops are fighting the Islamists.

"An attack like this one keeps them in the news. It's about extracting maximum publicity. Al-Shebab is on its back foot in Somalia, it is not as strong as it used to be," said Mr Alex Vines, the head of the Africa programme in the London-based think-tank Chatham House.

The group was forced out of the Somali capital Mogadishu in August 2011 and left the key port of Kismayo in September the following year.

"This shows that they still maintain capabilities and can execute a high-profile terrorist attack outside the frontiers of Somalia," Mr Vines told AFP.

Many said it was inspired by the November 2008 attacks in India's financial heart and film capital Mumbai, when about 10 gunmen spread terror over four days and killed at least 166 people.

And every second that the Shebab fighters maintained a siege of the upscale Westgate shopping centre, popular with wealthy Kenyans and expatriates, ensured them the publicity they want, experts said.

A Shebab commander, who identified himself as Abu Omar, told the BBC on Monday that there was no question of negotiating with Kenyan authorities.

He said the members of the suicide mission had not entered the mall to return alive, adding they "wanted to die in the name of Allah to become martyrs".

Lieutenant Colonel Frederic Gallois, the former head of the National Gendarmerie Intervention Group, a special operations unit of the French Armed Forces, said the siege through its duration would "have a more terrifying effect than a bombing". The challenge for Kenyan security forces was to try to extricate the maximum number of hostages alive while minimising the dramatic effect of the attack.

Others said the modus operandi of extremists had changed since the 1970s when strikes basically involved hijackings.

Evoking the Mumbai attacks, Dr Vanda Felbab-Brown from the Centre for 21st Security and Intelligence of the Washington-based Brookings Institution, said: "There is a very powerful evidence that such operations motivate other terrorist groups to pick up the same kind of methods.

"In the 70s there was a lot of hijacking of ships and planes, then just about every terrorist group was trying to do that," Dr Felhab-Brown said.

"This mall was a very soft target... it's a visible target in a posh, prestigious neighborhood with a lot of foreigners going to it, it's the symbol of Western opulence in Kenya and there was no security. Just some unarmed people at the entrance, looking into bags and running a metal detector over you. Very easy target," she said.

Kenyan Red Cross officials said on Monday that 62 people had died in the attack on the partly Israeli-owned shopping centre.