The United Nations aid chief vowed Friday to "move mountains" to rescue those stuck under the rubble after deadly twin earthquakes struck Venezuela, with dozens of international teams joining the anguished search for survivors.

The situation is "just devastating", Tom Fletcher, the UN Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator, told AFP in an interview in Geneva.

The two earthquakes, which struck within a minute of each other on Wednesday night, flattening buildings in the north of the country, have killed at least 920 people, according to an official updated toll.

And Fletcher said that more than 50,000 people were missing, warning that it would be "a massive job to go through the rubble".

He said the UN humanitarian agency OCHA, which he runs, did not yet have an estimate for how high the death toll in Venezuela was likely to climb.

But, he said, "50,000 people are missing... It's clearly going to rise significantly".

The coastal area of La Guaira, near the capital, is the hardest hit, with one building after the other crumpled into dust and rubble by the quakes, measured at magnitudes 7.2 and 7.5.

Family members, neighbours and volunteers resorted to their bare hands to try to claw out survivors, pointing to the lack of heavy machinery or official help to save those trapped alive.

"The focus for now has to be finding the survivors and saving as many people as possible," Fletcher said.

He said he had been deeply moved by accounts from people "telling us that they can hear the voices".

"That's the unimaginable thing... You can hear the voices, you know they're there," he said.

"Every minute counts."

The UN aid chief said he wanted "people with families under the rubble, people whose loved ones, friends, and families are there... to know that we... are going to do our best". 

"We're going to move mountains to save their loved ones."

Fletcher acknowledged though that  "it's a very, very complex emergency response", pointing out that amid the desperate rush to save lives, "the threat of aftershocks... (means) the rescuers are going out into amazingly complex conditions".

International rescuers and aid have begun arriving to help a nation struggling to respond.

Fletcher said the operation underway was "massive", with the local teams leading the effort joined by dozens of international teams.

"We've got 35 search and rescue teams now deploying already on the ground," he said, pointing out that they jointly counted "over 1,600 trained urban rescue colleagues (and) over 100 rescue dogs".

And Fletcher said he expected even "more will be mobilised for this first wave". 

He hailed the "enormous mobilisation" and the solidarity on display, saying he had been in contact with rescue teams "from across the region, from across Europe, from Qatar, from Jordan... and the Syrians are sending a team as well".

The United States has also sent teams and has said it is deploying two warships, transport planes and helicopters, and was mobilising $150 million in aid for the quake-hit country.

On Friday, Washington also announced that it had lifted a number of economic sanctions against Venezuela for four months to facilitate relief operations.

Fletcher said that was good news, pointing out that "wherever there are sanctions, we need humanitarian exemptions" to bring aid in".

"You want to make sure that there's nothing that prevents us getting those essentials to those who need them."

Fletcher said OCHA was playing a key role in the challenging coordination of the response, helping channel the "huge amount of help that people want to give".

The agency had set up a coordination cell on the ground in Caracas, where it was coordinating between the UN-led effort and the local authorities.

And it had an analysis cell at its headquarters in Geneva, which Fletcher said was "bringing together all of the data... so that we can make sure our teams are deployed in the right place and in the right way".

The agency was also working on estimating the funding needs for the response, he said, pointing out that even before the quakes hit, some eight million people in Venezuela were in need of humanitarian aid.

"Obviously, that number has now gone up dramatically," Fletcher said, calling for countries and individuals alike to pitch in.

© Agence France-Presse