WASHINGTON (AP) — Parents across the U.S. are scrambling to find baby formula because supply disruptions and a massive safety recall have swept many leading brands off store shelves.

Months of spot shortages at pharmacies and supermarkets have been exacerbated by the recall at Abbott, which was forced to shutter its largest U.S. formula manufacturing plant in February due to contamination concerns.

On Monday, White House press secretary Jenn Psaki said the Food and Drug Administration was “working around the clock to address any possible shortages.”

On Tuesday, the FDA said it was working with U.S. manufacturers to increase their output and streamlining paperwork to allow more imports.

For now, pediatricians and health workers are urging parents who can’t find formula to contact food banks or doctor's offices. They warn against watering down formula to stretch supplies or using online DIY recipes.

“For babies who are not being breastfed, this is the only thing they eat,” said Dr. Steven Abrams, of the University of Texas, Austin. "So it has to have all of their nutrition and, furthermore, it needs to be properly prepared so that it’s safe for the smallest infants.”

Trying to keep formula in stock, retailers including CVS and Walgreens have begun limiting purchases to three containers per customer.

Nationwide about 40% of large retail stores are out of stock, up from 31% in mid-April, according to Datasembly, a data analytics firm. More than half of U.S. states are seeing out-of-stock rates between 40% and 50%, according to the firm, which collects data from 11,000 locations.

Baby formula is particularly vulnerable to disruptions because just a handful of companies account for almost the entire U.S. supply.