New Zealand Prime Minister John Key has declared a national state of emergency as the death toll from Tuesday's earthquake in Christchurch rose to 75.
Police have said there is "incredible carnage right throughout the city", with "bodies littering the streets".
More than 300 people are still missing. Forty-eight were pulled out from collapsed buildings alive overnight.
The search has now been abandoned at one office block, but elsewhere a woman was rescued after more than 24 hours.
"The situation is that we don't believe this site is now survivable," said police area commander Inspector Dave Lawry near the remains of the Canterbury Television building. "It was a hard choice and my heart goes out to all the families."
He said rescuers now needed to concentrate their resources elsewhere in areas where survivors were more likely to be found.
About 100 people are believed to be inside. Earlier, the head of the fire service denied reports that 15 people trapped inside had been rescued.
The building housed a language school and Insp Lawry said some foreign students would be among the dead there.
At the Pyne Gould Guinness building, office worker Ann Bodkin was pulled out alive on Wednesday afternoon, after being trapped for more than 24 hours, and was reunited with her waiting husband.
The authorities have imposed a night-time curfew in the worst-affected areas of the city.
The earthquake struck at a shallow depth of 5km (3.1 miles) on Tuesday lunchtime, when the South Island city was at its busiest.
It was Christchurch's second major tremor in five months, and New Zealand's deadliest natural disaster in 80 years.
Rescue priority
Prime Minister John Key, who grew up in Christchurch, declared a national state of emergency after a cabinet meeting in the capital, Wellington. He said it would allow the greatest possible co-ordination of local, national, and international resources to work on rescue and recovery efforts.
He told residents of Christchurch that New Zealanders felt "your pain as only a small nation can".
"Things will get better. Christchurch will rise again," he added. "This country is right behind you and we are backing you with all of our might."
Mr Key ordered that the country's flag be flown at half-mast on all public buildings until further notice to honour the victims.
He said he he hoped that the city would still be able to host its allotment of games for the 2011 Rugby World Cup in September-October.
"It's a very important city to New Zealand and it would be a demonstration Christchurch is back up on its feet."
But the head of the cup's organising committee said it was too early to make such a decision.
"Any assessment by us must wait while the rescue efforts take priority," said Rugby New Zealand 2011 chief executive Martin Snedden.
He said no games would be shifted offshore, to Australia.
'Incredible carnage'
More than 500 search and rescue personnel, police, fire service staff, soldiers and volunteers have been scrambling to find survivors trapped under the rubble, many using only their bare hands.
"There is incredible carnage right throughout the city," Police Superintendent Russell Gibson said. "There are bodies littering the streets, they are trapped in cars and crushed under rubble."
"We are getting texts and tapping sounds from some of these buildings and that's where our focus is," he added.
"It's quite amazing, we have some people we've pulled out and they haven't got so much as a scratch on them, we've had other people where we've had to amputate limbs to get them out."
Officials said a total of 300 people were believed to be missing, but details are unclear and officials are currently trying to refine that list. Some people may simply not have been able to contact friends and relatives.
The ministry of civil defence said 22 people alone were missing in Christchurch Cathedral, which lost its spire and a section of roof. Police say there has been no sign of life from underneath the rubble.
Twenty-four others have meanwhile been rescued from the Pyne Gould Guinness building and dogs have detected another four or five still alive. The earthquake flattened the four-storey structure where hundreds worked.
"I rang my kids to say goodbye," Ann Voss told TV3 from underneath her desk inside the building. "It was absolutely horrible. My daughter was crying and I was crying because I honestly thought that was it."
Ms Voss said she could hear other survivors and had called out to them.
"I'm not going to give up," she added. "I'm going to stay awake now. They better come and get me."
Rescuers have also put a cordon around the city's second tallest building, the Hotel Grand Chancellor, because it is threatening to collapse. The hotel has been displaced by 0.5m (1.6ft) and has dropped by 1m (3.3ft) on one side.
Emergency shelters have been set up at the city's Hagley Park, a race course, schools and community halls.
The Red Cross has been trying to find accommodation for people sheltering outside in tents or under plastic sheeting.
All the schools in Christchurch are closed until further notice, as expert teams are assessing any potential damage to the buildings.
'Utterly shocked'
Britain's Queen Elizabeth II, who is also New Zealand's head of state, said she was "utterly shocked" by the disaster, while US President Barack Obama offered his "deepest condolences".
The damage is thought to be far worse than after the 7.1-magnitude quake on 4 September, which left two people seriously injured but no fatalities.
The epicentre of that quake, which occurred in the middle of the night, was further away from the city and deeper underground.
New Zealand experiences more than 14,000 earthquakes a year, of which only around 20 have a magnitude greater than 5.0.
The last fatal earthquake was in 1968, when a 7.1-magnitude tremor killed three people on the South Island's western coast.
Tuesday's was the country's worst natural disaster since a 1931 quake in Hawke's Bay on the North Island killed 256 people.
