A curfew has ended in Chile's second city, Concepcion, with thousands of troops now trying to prevent a renewal of looting in the earthquake zone.
Chilean President Michelle Bachelet says 14,000 troops are now in the region, after dozens of people were arrested for looting on Monday.
A special air route is being set up to deliver aid from the capital, Santiago, to Concepcion, 430km (270 miles) away.
An 8.8-magnitude earthquake early on Saturday killed at least 723 people.
The deteriorating security situation in Concepcion comes despite the influx of thousands of troops to reinforce local police.
Many of the city's 500,000 inhabitants are short of food and have seen their water and electricity supplies cut off.
But lorry after lorry loaded with water, food and mattresses is being held up by the military until the curfew is lifted, reports the BBC's Andy Gallacher from the checkpoint on the outskirts of Concepcion.
The main highway in the region is twisted and bent out of shape, but the route remains open, our correspondent says.
Security seems to be the biggest issue holding up rescue efforts, he adds.
Some residents quoted by Reuters news agency said they were organising groups to defend their property.
President Michelle Bachelet sent the troops to the region condemning "pillage and criminality".
"We can say that, according what we've been told from the area, the situation in Concepcion is under control today," she said.
But, she added, authorities would take any "necessary measure" to stop renewed looting.
"Our principle objective is to go and help people tackle the emergency in the disaster zone. I want them [looters] to understand this and that they'll receive rigorous legal action. We will not tolerate such actions."
Meanwhile, rescuers searching the rubble of a collapsed apartment building in the city in which dozens are feared trapped say they have heard signs of life and are attempting to reach survivors.
Reports are beginning to emerge of the scale of the devastation in other areas.
Up to 90% of the mud-and-wood buildings in the historic centre of Curico had been destroyed or damaged, and a hospital badly damaged, BBC reporters said.
Some coastal towns and villages were also hit by giant waves after the earthquake, with some reported to have been completely destroyed.
The government admits that its attempts to provide aid swiftly have been hampered by damaged roads and power cuts. The air supply route between Santiago and Concepcion will help the authorities send more than 300 tonnes of aid, including 120 tonnes of food, to the worst affected area of the country.
International aid has begun arriving. Neighbouring Argentina is flying a field hospital over the Andes to Chile and has pledged half a million litres of much-needed drinking water.
Brazilian President Luiz Inacio Lula da Silva flew to Santiago and offered his nation's support, as did US Secretary of State Hillary Clinton.
Mrs Clinton took a consignment of satellite phones with her to Santiago after the Chilean government requested communications equipment alongside field hospitals and water purification units.
"One of their biggest problems has been communications," Mrs Clinton told reporters. "They can't communicate into Concepcion and some of the surrounding areas."
After touring the disaster zone, President-elect Sebastian Pinera said the situation was worse than he had expected.
"When we have a catastrophe of this magnitude, when there is no electricity and no water, the population... starts losing the sense of public order," he said.
About two million Chileans are believed to have been affected by Saturday's earthquake, the seventh most powerful on record and the worst disaster to befall Chile in 50 years.
The epicentre of the quake was 115km (70 miles) north-east of Concepcion and 325km south-west of the capital Santiago.
About 1.5 million homes in Chile have been damaged. Most of the collapsed buildings were of older design - including many historic structures.
One US risk assessor, Eqecat, has put the cost of repairing the damage at between $15bn and $30bn (£9.8bn - £19.6bn) or 10-20% of the country's gross domestic product.
Source : bbc