Rescuers listened intently for sounds of survivors trapped under the wreckage of buildings collapsed by Chile's deadly earthquake Monday, while hundreds of homeless victims waited for aid and authorities struggled to control looters in the area hit hardest by the disaster.

In the central Chilean city of Concepcion, the largest urban area near the epicentre of the 8.8-magnitude earthquake, rescue workers heard the faint knock of trapped victims inside a toppled 70-unit apartment building and immediately demanded silence from everyone within a one-block radius.

Only the chop of military helicopters passing overhead broke the silence demanded by the rescuers straining to hear signs of life inside the building.

They quickly began to drill through thick walls to reach those trapped inside, said Fire Department Commander Juan Carlos Subercaseux.

Firefighters had already pulled 25 survivors and nine bodies from the structure since Saturday's earthquake leveled it, one of an estimated half-million homes destroyed in the quake.

The death toll rose to 723 Monday, with 19 others missing, according to Chile's National Emergency Office, creating what President Michelle Bachelet called "an emergency without parallel in Chile's history."

Some coastal towns were almost obliterated in the disaster -- first shaken by the quake, then struck by a tsunami that carried whole houses inland and crushed others into piles of sticks and rubble.

Concepcion Mayor Jacqueline van Rysselberghe told Radio Cooperativa that some food aid was arriving in the city of 200,000 Monday for distribution to survivors who have been without power, water or food since the quake.

The UN said Monday that it would rush emergency shipments to Chile in response to Bachelet's appeal for international aid. UN humanitarian spokeswoman Elisabeth Byrs said Chile was seeking temporary bridges, field hospitals, satellite phones, electric generators, damage assessment teams, water purification systems, field kitchens and dialysis centres.

"We are prepared to provide assistance," Byrs told The Associated Press in Geneva. "It could be quite fast, given that our experts are on standby and were alerted in the region."

In Canada, the Foreign Affairs department said in a statement that it was standing by to give Chile whatever help it requests.

"Canada stands ready to provide any necessary assistance to the people of Chile during this time of need," the department said on its website.

The department said 520 Canadians known to be in Chile had been contacted as of Monday afternoon, while 337 others had not yet been located.

U.S. Secretary of State Hillary Rodham Clinton also offered to provide disaster aid. Travelling in Uruguay, Clinton said she would bring some communications equipment when she visits Chile on Tuesday.

Argentina said it was sending six aircraft loaded with a field hospital, 55 doctors and water treatment plants.

Bachelet ordered troops to help deliver food, water and blankets and clear rubble from roads, and she urged power companies to restore service first to hospitals, health clinics and shelters. Field hospitals were planned for hard-hit Concepcion, Talca and Curico.

Bachelet also ordered authorities to quickly identify the dead and return them to their families to ensure "the dignified burials that they deserve."

The World Health Organization said it expected the death toll to rise in the coming days as communications improve. For survivors, it said access to health services will be a major challenge and noted that indigenous people living in adobe homes were most at risk from heavily damaged infrastructure.

Electricity was still out in the region hit most severely by the quake, water was scarce and at dusk looters re-emerged despite beefed up security and a curfew enforced by police.

Dozens of people sacked stores selling food, clothing and drugs, fleeing whenever police appeared to drive them away. Some struck gas stations, stealing cash from attendants.

Canadian freelance journalist Jen Ross said the area is in a "virtual state of chaos."

"Desperation is setting in, as people have no water, no food," Ross told CTV News Channel in an interview Monday afternoon. "And the military is called in but the mayor there is making extremely urgent pleas to send in more (troops) because they say that it's not enough."

Police chief Eliecer Soler said officers arrested 55 people for violating a nighttime curfew imposed after looters sacked nearly every market in town Sunday. Troops ordered into the city by Bachelet patrolled to enforce security. A few looters re-emerged to rob a market on Monday.

As a small military convoy of drove down the main avenue of Concepcion Monday, bystanders applauded and shouted: "Finally! Finally!"

Spanish professor Eduardo Aundez watched with disgust as a soldier patiently waited for looters to rummage through a downtown store, then lobbed two tear gas canisters into the rubble to get them out.

"I feel abandoned," he said. "We believe the government didn't take the necessary measures in time, and now supplies of food and water are going to be much more complicated."

In Concepcion the ground started rumbling at 3:34 a.m. on Saturday. It lasted only 90 seconds, but caused widespread damage.

More than 500 kilometres north of Concepcion, in the Chilean capital of Santiago, buildings and bridges were damaged.

Nathan Crooks, an American editor with Business News Americas, was sleeping in his Santiago apartment when the rumbling started.

"The whole building began to shake and there was an incredible noise outside, it sounded like a freight train," Crooks told News Channel by telephone Monday morning.

"About halfway through the quake, all the power to the city got cut off and it was dark and it kept shaking. Things began to fall. And it just went on so long -- that's what was so surprising about the quake. We're used to earthquakes in Chile, but this one, it was just very strong and went on and on and on."

The rumblings from Saturday's quake were felt as far away as Sao Paolo, Brazil, nearly 3,000 kilometres away.

Defence Minister Francisco Vidal acknowledged the navy made a mistake by not immediately activating a tsunami warning after the quake hit before dawn Saturday. Port captains in several coastal towns did, saving what Vidal called hundreds of lives.

Thirty minutes passed between the quake and a wave that inundated coastal towns, leaving behind sticks, scraps of metal and masonry houses ripped in two. A beachside carnival in the village of Lloca was swamped in the tsunami. A carousel was twisted on its side and a Ferris wheel rose above the muddy wreckage.

A series of aftershocks collapsed more buildings and forced thousands to set up tents in parks and highway medians."If you're inside the house, the furniture moves," said Monica Aviles, when speaking to The Associated Press.

When an aftershock struck moments later, Aviles said "that's why we're here."

Crooks said the many dozens of aftershocks that followed the quake have paled in comparison to the rumblings that hit Saturday morning.

"I don't really even notice them any more, you kind of hear the walls in your house start to creak a little bit, but the aftershocks, they're nothing compared to the real quake," said Crooks.

With files from The Associated Press