The nation's largest nurses union condemned the federal government's emergency response in Puerto Rico on Thursday for "delaying necessary humanitarian aide to its own citizens and leaving them to die."

The stinging criticism came from members of the nonprofit National Nurses United, speaking on Capitol Hill with Democratic members of Congress after a two-week humanitarian mission to Puerto Rico. About 50 volunteer nurses visited two dozen towns in urban and rural areas, and described the desperation of Puerto Ricans — even five weeks after Hurricane Maria hit the island — as worse than anything they had witnessed on other humanitarian missions, including the aftermath of Katrina in New Orleans and the earthquake in Haiti.

The official death toll from the storm so far is 51, though Vox's own reporting suggests the actual number of deaths could be in the hundreds.

The nurses described doctors performing surgery in hospitals with light from their cellphones, children screaming from hunger, elderly residents suffering from severe dehydration, and black mold spreading throughout entire communities.

"We cannot be silent while millions of people continue to endure these conditions," said Bonnie Castillo, associate executive director of National Nurses United.

She said some nurses arrived in towns that never got food or water supplies, or any other help from the Federal Emergency Management Agency. Some communities the nurses visited that did get supplies were not getting enough.

It's hard to get information about overall relief efforts in Puerto Rico, as some towns remain completely cut off from internet and phone service. Right now, about 75 percent of the island doesn't have electricity and 25 percent doesn't have running water. (Vox's reporting suggests that far fewer people have access to drinking water than statistics show.)

FEMA has acknowledged that its mission in Puerto Rico is the most "logistically complex response in FEMA history," while also pointing out how much it has accomplished in such a difficult environment. The agency has deployed more than 16,000 federal workers to the island, flown supplies to stranded communities more than 700 times, and delivered more than 10 million liters of bottled water. The agency has also approved more than $114 million in individual assistance to cover the cost of temporary housing and other basic needs. It has also approved $2.3 million in disaster assistance loans for small businesses.

But Olivia Lynch, a California nurse who just returned from the Puerto Rico mission, said FEMA’s efforts in Puerto Rico are "completely inadequate."

Part of the problem is how FEMA requires Puerto Ricans to apply for disaster aid. Lynch said FEMA workers were asking residents to give them copies of utility bills and bank routing numbers. Then the agency promised to follow up via email or text. But most people don't have electricity or phone service, and their homes and documents may be destroyed.

The nurses union said it had put out a report about its findings in Puerto Rico but the FEMA administrator in Puerto Rico has yet to return the union’s calls to discuss them.

Here is what most concerned them from their experience on the ground:

People standing in line for hours in the sun for food and water, with federal workers giving them paperwork instead of distributing supplies

Residents living in soaked homes without roofs, where dangerous black mold is spreading and leading to respiratory problems

Rural towns that have never gotten food and water supplies, and yet have no running water and no electricity

An outbreak of leptospirosis, a dangerous bacterial disease that has already claimed lives; as of Thursday, four deaths have been attributed to this outbreak

Multiple communities without clean water that are at risk of the outbreak of water-borne illness epidemics

Kathy Kennedy, a nurse who once worked on the military medical ship USNS Comfort, said she couldn't believe there were still empty hospital beds aboard the ship, which is docked near San Juan. She said hospitals on the island can barely take care of their patients and need more help.

The nurses and members of Congress asked Republican leaders to increase disaster aid to Puerto Rico, and to prioritize and spend more money on repairing the power grid.

"Our people are being left to suffer, and the nurses hope that our elected officials work to change this before people die," Kennedy said.