Shoppers are buying six-month food supplies wrapped in military-grade Mylar pouches, and kits filled with duct tape, food bars and an air-filtration mask.

Some companies that specialize in selling items to people planning for the worst — so-called doomsday preppers — say they have had a bump in sales this week, after tensions rose between the United States and North Korea. Online searches for prepping and survival gear have also jumped.

The increase in sales is a turnaround from recent months. Revenue fell after Mr. Trump’s election, according to three of the country’s larger retailers that specialize in selling emergency preparedness.

“The whole industry kind of took a little pause after the election of Trump,” said Brandon J. Garrett, director of marketing at The Ready Store, which sells a wide range of prep products online and via catalog. “I think everyone was kind of waiting to see what kind of leader he was going to be and where he would take the country.”