CARACAS — The United States banned all air transport with Venezuela on Wednesday over security concerns, further isolating the troubled South American nation by severing one of its last links to the world’s largest economy.

The Department of Homeland Security said it decided to immediately suspend all commercial and cargo flights between the United States and Venezuela because the country’s political crisis threatened the safety of passengers, aircraft and crew.

The decision will be a heavy blow for millions of Venezuelans who rely on donations or remittances from relatives abroad to survive, as the country’s crumbling economy has destroyed most of its industry and agriculture and slashed government imports. Many have relied on airline courier services from Miami to obtain scarce medication, spare parts and food.

“This will be a catastrophe for a lot of people,” said Feliciano Reyna, head of the health nonprofit Acción Solidaria, which receives medical donations from the United States through air courier services. “This will complicate enormously the transportation of humanitarian aid to the country.”