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.
More than 500 search and rescue personnel, police, fire service staff, soldiers and volunteers worked throughout the night 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."
Asked how many may still be trapped, Supt Gibson said: "It could be another 100 - it could be more."
Later, 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.
Amid scenes of devastation in the Cashel Street Mall, an injured baby was found in its dead mother's arms, witnesses said.
Tom Brittenden, who worked nearby, said the mother had run out of a shop during the earthquake and been hit by debris.
"We tried to pull these big bricks off her," he said. ''It was a big 1m by 1m cube which hadn't separated... She was gone. Some people who were helping took the baby away. They just put a blanket on her because she had already gone.''
Earlier, officials said 55 bodies had so far been identified around the city and that 20 remained unidentified.
Supt Gibbs said the death toll would be "significantly higher". The police are aware several locations, including a bus crushed by debris, where bodies have not yet been removed because their priority is to help those still alive.
Police Superintendent Dave Cliff told the BBC that there was a fleet of air ambulances and military aircraft standing by to take people to other hospitals around the country.
"They haven't needed to be used yet, because the hospital system here across Canterbury is coping with the numbers. But that's all ready to go as people who need that trauma care are pulled out of rubble," he told the BBC.
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.
Prime Minister John Key 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 also ordered that the country's flag be flown at half-mast on all public buildings until further notice to honour the victims.
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 in excess of 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.