LA GUAIRA : Hopes were fading Monday of finding survivors more than four days after powerful twin earthquakes struck Venezuela, as residents grow increasingly frustrated with the government's response to the disaster that has killed at least 1,450 people.

French and American rescue teams found a man and his teen son alive under the rubble on Sunday in Caraballeda, a town about 40 kilometers (25 miles) north of Caracas, AFP journalists saw.

The rescue offered a glimmer of hope in an ongoing tragedy that has shaken a country already mired in an economic crisis, but tens of thousands of people were still unaccounted for as the critical 72-hour window for rescuing trapped victims passed.

Millions more people were feared to lack sanitation and other basic needs after one of Latin America's most devastating earthquake disasters.

Some 774 buildings were badly damaged in back-to-back quakes of magnitude 7.2 and 7.5 that struck on Wednesday evening, including 189 buildings that have totally collapsed, National Assembly President Jorge Rodriguez said on Sunday.

In the coastal town of Tucacas, rescuers were digging for people trapped in the pancaked layers and rubble of a collapsed building complex.

Luis Salas, 27, who joined the rescue efforts, told AFP that "the hardest part was when we felt hope in the tunnels we went into -- crawling, clearing debris, working with all our heart, with great faith -- and when we reached our targets, we found them lifeless."

Experts say the first 72 hours after natural disasters define the narrow window for rescuing the living. After that the search usually becomes one of recovering bodies.

In the capital's San Bernardino neighborhood, volunteers clambered over a collapsed building, using drills to break up concrete and forming lines to remove rubble by hand.

In Chacao, another area of Caracas, large electronic screens on a building usually used for advertising were showing the faces of missing people.

On Sunday, Rodriguez said the death toll -- which was still expected to rise -- had reached 1,450 people, with at least 3,150 others injured.

- 'Can't do it alone' -

In one of the worst-hit areas, Hector Aguilera came to search for four family members buried in the rubble in the coastal city of La Guaira.

"We don't have the support to get our family out -- we can't do it alone. They are buried there: we know they are dead, but here we are," he said.

Even as rescue efforts continued, public anger has mounted in some areas.

Eduardo Cardozo, a volunteer in Tucacas, said it was "frustrating" to know that some victims could have been saved "if they'd been searched for in time."

In La Guaira state's Tanaguarena area, one man urged soldiers to pick up picks and shovels: "The country needs you. Put down your weapon."

Outbreaks of looting have hit La Guaira city, much of which now lies in rubble.

Pharmacies, supermarkets and other businesses were ransacked, said residents, some of whom complained of the slow and meager post-quake aid coming from authorities.

Venezuela's interim president, Delcy Rodriguez, said on Sunday that temporary camps were being set up for people who had lost their homes.

"At the same time, work begins on planning projects that will allow new homes to be built in a very short time," she said.

- Economic impact -

Rodriguez praised rescuers on Sunday, saying "we have rescued people who are still alive, and therefore these efforts will not be suspended."

"We always hold onto hope."

Cardozo, the Tucacas volunteer, remained hopeful: "We're still here waiting. Let's see if we can get someone else out."

Twenty-four nations have sent 521 tons of supplies, 86 units with dogs trained to locate people trapped beneath the rubble and more than 2,700 search-and-rescue personnel, according to Rodriguez.

US helicopters ferried in aid, and 230 more US military personnel were arriving to help expand airport capacity and reopen a key seaport to boost relief efforts, the US Southern Command said.

The United States -- which captured Venezuela's former president Nicolas Maduro in a military raid on Caracas in January -- had already sent a 250-strong disaster response team.

The UN migration agency said that based on population and damage data, up to 6.76 million people could be affected, and would require shelter, water, sanitation, healthcare and essential relief items.

Venezuela's worst earthquakes in more than a century have come after the oil-rich country endured more than a decade of economic collapse.

The crisis has hollowed out hospitals and public services, driving millions to leave the country.

The United Nations estimated $6.7 billion in physical damage -- equivalent to six percent of Venezuela's GDP.