With the death toll expected to rise significantly, Kenyan security forces were still battling up to a dozen Somali killers of an al-Qaeda linked terrorist group, al-Shabaab, as the siege entered its third day. The surviving terrorists were believed to be holed up with an unknown number of hostages on the third floor of the four-storey Westgate shopping centre after a series of gun battles. They threatened to kill the hostages if security forces did not back off. Ross Langdon and his partner Elif Yavuz: they were expecting their first baby in two weeks. Credit:Peter Adams/www.windgrove.com ''We authorise the mujahideen inside the building to take actions against the prisoners as much as they are pressed,'' an al-Shabaab leader, Ali Mohamud Rage, said in a statement posted on an Islamic website. Kenyan President Uhuru Kenyatta said security forces had rescued more than 1000 people from the centre since the slaughter began on Saturday, and he vowed the terrorists would pay for their crimes.

''They shall not get away with their despicable and beastly acts,'' the President told his nation. ''We will punish the masterminds swiftly and indeed very painfully.'' Mr Kenyatta revealed that a nephew and his nephew's fiancee had been killed. Died in Kenya: Ross Langdon. Credit:http://architectureau.com/ The militant group said the attack was in retaliation for Kenya's military intervention in Somalia. ''We'll not negotiate with the Kenyan govt as long as its forces are invading our country, so reap the bitter fruits of your harvest,'' al-Shabaab said on a new Twitter feed, after a previous one was shut down.

Elif Yavuz worked for the Clinton Foundation in Kenya, and in that role had been visited last month by former US president Bill Clinton. Credit:Facebook Kenyan forces entered Somalia two years ago to fight al-Shabaab, and stay on as part of an African Union force supporting Somalia's internationally backed government. The weekend's attack is the worst in Nairobi since an al-Qaeda bombing at the US embassy in 1998 killed more than 200 people. Ross was just this true angel of light ... to lose that spark of positiveness leaves me grief stricken The Australian government declared al-Shabaab a terrorist organisation in August 2009. Several weeks beforehand, five Australian men with alleged links to the organisation were charged with planning a suicide attack on a the Holdsworthy army base in Sydney.

Three of those men were found guilty and sentenced in 2011 to 18 years in jail. In sentencing the men, Justice Betty King said: ''The fact that Australia welcomed all of you, and nurtured you and your families, is something that should cause you all to hang your heads in shame that this was the way you planned to show your thanks for that support.'' Melbourne woman Sarah Williamson, 23, survived the Nairobi siege when she took shelter in a telephone shop, where she huddled for several hours. Ms Williamson was in the shopping centre with her father, who works for the United Nations, when she heard the first explosions. Her mother Linda told Fairfax Radio Network: ''She heard the first grenade go off and said, 'What was that, dad?' ''Then he heard the guns firing and he knew exactly what it was and he said: 'We're under attack, everybody to the back of the room.'''

It is unclear where Mr Langdon and Ms Yavuz were when they were killed. Mr Langdon's family in Tasmania was told on Monday that he and Ms Yavuz had both died in the terrorist attack. Mr Langdon, 32, was a founder of the prize-winning architecture firm Regional Associates, which has offices in Melbourne, London and Uganda, where they first met. A committed conservationist, he led all the firm's projects in East Africa; was completing an HIV-AIDS clinic in Uganda - which he designed without charge - and was about to start on a $35 million museum telling the story of the earliest fossil record of walking humanoids in Kenya. Ms Yavuz, a specialist in malaria, worked with the Clinton Foundation in Tanzania. Last month she was paid a visit by former US president Bill Clinton.

The couple had returned to Nairobi from their respective projects to ensure Ms Yavuz got reliable medical care for the birth of their child. Melbourne-based Regional Associates partner Ben Milbourne said Mr Langdon and Ms Yavuz both had a deep passion for helping people, particularly in highly disadvantaged areas, through different means. ''They greatly believed in the power of their work to assist people,'' he said. Artist Peter Adams, of Nubeena on the Tasman Peninsula where Mr Langdon was born, wrote on his website that he had burst into tears when he heard of Mr Langdon's death. ''He was a colleague and friend who went out into the world as an architect doing wondrous things,'' Mr Adams wrote. ''Yet Ross always returned to his family and cultural roots here on the Tasman Peninsula and we all took immense pride in both his architectural abilities and his very generous, positive, and loving personality. There just was no dark side to Ross that I ever saw in the 20 or so years I knew him.'' Tasmanian premier Lara Giddings condemned the terrorist attack in which Mr Langdon and his partner died, saying it had sent shock waves across the world.

''I can only imagine the grief that this man's relatives and friends are experiencing at such a senseless attack that has claimed innocent victims, including this man and his partner,'' Ms Giddings said. Prime Minister Tony Abbott said in a statement: "That an Australian was among those killed in the attack is a terrible reminder that Australia is not immune from acts of terrorism around the world and that al-Qaeda-linked groups continue to present a serious global threat." Mr Langdon's relatives in Hobart have asked for their privacy to be respected. Loading Correction: The original version of this story said Elif Yavuz worked for the Clinton Health Access Initiative. She worked for the Clinton Foundation.