Rescue teams are racing to Venezuela’s shattered northern coast after almost simultaneous earthquakes reduced dozens of buildings to rubble, killing at least 235 people but with thousands more fatalities feared.

The US secretary of state, Marco Rubio, said the defence department would help search and rescue teams deploy to the affected region after Venezuela’s main gateway, the Simón Bolívar international airport, near the capital, Caracas, was badly damaged by 7.2 and 7.5 magnitude quakes less than 40 seconds apart, late on Wednesday afternoon.

He said the most immediate need was search and rescue. “They have [lots of] collapsed buildings and so they will need a lot of help in terms of digging through that,” Rubio told reporters, adding that the next 72 “golden” hours were critical.

“In search and rescue you are trying to get to people while you can still save their lives – they are buried under rubble.”

The coastal area near the international airport, around the cities of La Guaira, Catia La Mar and Caraballeda, appears to have sustained by far the worst damage, with a string of large tower blocks levelled and people desperately hunting for missing loved ones. In some cases families of four or five people have disappeared.

“This is an utter tragedy,” the acting president, Delcy Rodríguez, said in a televised broadcast, declaring the La Guaira region a disaster zone.

Caracas also sustained severe damage, with several buildings collapsing in the Altamira and Los Palos Grandes neighbourhoods. On Thursday evening the official death toll was raised to 235, with an earlier count putting the number of people missing at 157.

Tom Fletcher, the head of the UN’s humanitarian agency, Ocha, said: “We are fully mobilised right now … We will surge in people, we will surge in solidarity and, most important, we will surge in search and rescue support … for people who have lost so much … Now is the time for action.”

The UN agency reported that more than 100 buildings had collapsed in the La Guaira region alone. They included a large block of flats called the Ritasol Palace and the seafront Eduard’s Hotel. Those missing include children as young as five as well as elderly people.

The quakes were so strong that they were felt in the Brazilian city of Manaus, in the Amazon, more than 1,000 miles to the south of Caracas, forcing people to flee their homes.

As aftershocks continued to shake northern Venezuela on Thursday, world leaders offered their condolences and support to a nation already reeling from years of economic and humanitarian crisis and political repression.

The French president, Emmanuel Macron, said: “France stands ready, alongside its European partners, to provide assistance to the affected populations … A team of 85 French specialised rescuers … will be deployed immediately.”

Brazil’s president, Luiz Inácio Lula da Silva, voiced “great concern and dismay” for the people of Venezuela, who had already shown “great resilience in the face of adversities”.

The US president, Donald Trump – who turned Venezuela’s political landscape on its head by ordering the abduction of its dictator, Nicolás Maduro, on 3 January this year – said: “The USA stands ready, willing, and able to help! I have instructed all agencies of our government to get ready to move quickly. We will be there for our new and great friends. Early reports are not good!!!”

Rodríguez, Venezuela’s former vice-president who took power with Trump’s blessing after Maduro’s downfall, expressed gratitude for the global outpouring of solidarity, writing on social media: “Venezuela will never forget the helping hand extended to our people during these difficult times.”

In a televised address, she said the region worst devastated by Wednesday’s “unprecedented seismic phenomenon” was the state of La Guaira, the capital of which bears the same name. “There are dozens of collapsed buildings there and right now we are engaged in the really arduous task of rescue work in the hope of saving the lives God will allow us to,” she said.

Aerial footage painted a devastating picture of the situation in La Guaira, which authorities consider the disaster’s “ground zero”. The sweep of Caribbean beach towns and resorts to the west of the airport lay in ruins, with many seafront buildings completely destroyed.