A day after two of the deadliest earthquakes in Venezuela's history, the race to save lives is becoming a race against time.

Rescue crews are digging through mountains of rubble, searching for survivors believed to be trapped.

GEOFF BENNETT: More than 900 people are now confirmed dead, and tens of thousands remain missing.

As international aid teams rush into the disaster zone, the true scale of this unfolding crisis is only starting to emerge.

Our Nick Schifrin starts our coverage tonight.

NICK SCHIFRIN: Today in Venezuela, the destruction is debilitating.

Rescue workers drill through the damage to find the living still pinned down two days later.

But, in this tragedy, so many did not survive.

The dead now line the road because the hospital morgue is full.

The number of missing is overwhelming.

Family members write the name of loved ones on a hospital door.

And the missing are also listed online.

This Web site includes more than 50,000 names, 83-year-old Ivan Martinez, siblings Crixander, Elvismar, and Antonella, Jahelys and her two children, Jorgelis and her son Jadiel.

The grief and desperation feels insurmountable.

Nazareth Jimenez's brother and his entire family are somewhere under their now-pancaked home.

NAZARETH JIMENEZ, La Guaira, Venezuela, Resident (through translator): My brother, my sister-in-law, my niece, my nephew, and friends are in there.

I got here yesterday at 4:00 in the afternoon, and nothing has happened.

They haven't cleared anything.

Where's the help?

I don't see it.

NICK SCHIFRIN: Others have lost everything.

Omar Reyes walks through the devastation and calls out for his missing wife and children.

There is no response.

And as he walks on the rubble in his sandals, there is no home left either.

OMAR REYES, La Guaira, Venezuela, Resident (through translator): More than 20 of my relatives have died.

I have been left practically alone in this world.

I am alone.

NICK SCHIFRIN: It is Venezuela's worst natural disaster in more than a century, twin 7.2 and 7.5 earthquakes that crumbled buildings, the scenes from the capital, Caracas, apocalyptic, and the most damage to La Guaira north to Caracas.

Entire blocks look like they have been blown apart, entire buildings collapsed, the state now placed under military control.

The damage appears worst closest to the coast.

It looks like the city's been bombed, buildings blown open.

The toll is immense.

Caracas' hospitals are now overflowing.

Many patients are being treated in the courtyard.

BEATRIZ OCHOA, Norwegian Refugee Council: I grew up in Mexico alongside the Pacific Ocean.

And I grew up with earthquakes.

And this was something I have never experienced before, the magnitude, but also the length.

NICK SCHIFRIN: Beatriz Ochoa is the Norwegian Refugee Council's Latin America head of advocacy.

BEATRIZ OCHOA: We just went under the frame of our bedroom.

It was not stopping.

And I just felt like, this is it.

NICK SCHIFRIN: Already, before the earthquake, nearly eight million Venezuelans, or more than one-quarter of the country, depended on humanitarian assistance, including food, water, and health.

Now people are sleeping on the street, either because they're homeless or too scared to sleep inside.

BEATRIZ OCHOA: People need a safe place to sleep.

People will need clean water.

We don't want diseases to be spread.

People need at this moment hot meals, and, of course, children particularly will need to recover and to have a sense of normality.

NICK SCHIFRIN: How much need is there right now in the immediate aftermath of the earthquakes?

BEATRIZ OCHOA: There is a lot of need.

The country was already struggling.

Recovering will be hard.

And that's why we need support from all over the world so that people can recover sooner, rather than later.

NICK SCHIFRIN: That support is beginning to show up.

Search-and-rescue teams from Spain, as well as the United States and a handful of other countries, arrived today.

And the U.S.

military is beginning to deliver $150 million worth of humanitarian assistance overseen by a two-star Marine general.

DONALD TRUMP, President of the United States: We're helping Venezuela.

They had a tremendous earthquake, a lot of people killed and unbelievable, right in Caracas.

And we have a lot of people over there helping.

NICK SCHIFRIN: State TV showed acting President Delcy Rodriguez handing out local aid.

But the needs are great.

Many places have received no help, and Venezuelans are overwhelmed by a wave of devastation.

For the "PBS News Hour," I'm Nick Schifrin.