Saturday is a "crucial day" for search and rescue efforts, says UN humanitarian chief Tom Fletcher in an interview with BBC Radio 4.
Rescue experts believe the first 48 to 72 hours are critical to the chances of finding survivors in the rubble.
"(The rescue workers) are being driven forward minute by minute, hour by hour, by the sound of survivors under the rubble," Fletcher said on the radio.
The worst thing is when those voices fall silent.
Shallow tremors
More than 900 people have been confirmed dead in the quakes - which had magnitudes of 7.2 and 7.5 - and thousands have been injured. But the actual death toll is feared to be much higher. More than 51,000 people have been reported missing.
The fact that the earthquakes were shallow contributed to the extensive damage to infrastructure and buildings.
In the hard-hit coastal region of La Guaira, just north of the capital Caracas, people are searching for their relatives themselves with simple tools, reports the AP news agency. Frustration over the lack of effective government efforts is great.
"We are appealing to the government and countries around the world for help," Nazareth Jiménez told the AP.
Enormous consequences
The UN migration agency IOM said nearly seven million people could be affected by the disaster, including two million in Caracas, AFP reported. The estimate highlights the disaster's "potentially enormous humanitarian consequences," according to a statement.
Due to the chaos and destruction in La Guaira, the Venezuelan authorities have deployed more than 14,000 soldiers and police to keep people away, the country's interim president Delcy Rodríguez said on state television on Saturday.
Several countries have sent emergency aid and rescue workers, including the United States, the United Kingdom, Chile, Colombia, El Salvador and Switzerland.
