WASHINGTON — Federal agencies grappled Saturday with the vast scale of the disaster in Puerto Rico left by Hurricane Maria, the third major storm to strike the U.S. in less than a month.

Three days after the massive hurricane crossed the U.S. territory, towns remained without fresh water, fuel, power or phone service.

The Federal Emergency Management Agency said meals and water and supplies were being delivered to the island and that some airports were reopening. But it was not clear how quickly relief would reach people, and fully restoring power and communications was expected to take some time.

“This is going to be a long and frustrating process for everybody. There is tremendous damage on the island,” said William Booher, director of public affairs for FEMA.

Booher said there was no difference in the agency’s response in Puerto Rico, compared with Texas, Louisiana and Florida after recent hurricanes. FEMA has had sufficient resources to deal with back-to-back-to-back hurricanes, he said, adding that “we’ve been able to address each one of them.”

Still, while FEMA officials train to respond to more than one disaster at once, three blows in the space of a few weeks offered a monumental challenge. Booher said there were about 5,000 emergency personnel in Puerto Rico, including some staff permanently assigned to the island.

President Trump has issued disaster declarations for Puerto Rico and the U.S. Virgin Islands. He visited Texas, Louisiana and Florida in the wake of hurricanes Harvey and Irma, but he had not yet traveled to Puerto Rico, though he has said he plans to make the trip.

“We’re going to start it with great gusto, but it’s in very, very, very perilous shape. Very sad what happened to Puerto Rico,” Trump said before a meeting at the U.N. General Assembly.

FEMA said Saturday that round-the-clock efforts were underway to help Puerto Rico and the U.S. Virgin Islands. Several airports were opened to help bring in goods and personnel. Ships and barges carrying meals, water, generators and other supplies were making their way to the islands, with more supplies being flown in. Sailors and marines in the region were conducting damage assessment flights, beach assessments and evacuating patients from a hospital in the U.S. Virgin Islands.

Meanwhile, in San Juan, a group of anxious mayors arrived to meet with Gov. Ricardo Rossello to present a long list of items they urgently need.

The north coastal town of Manati had run out of fuel and fresh water, Mayor Jose Sanchez Gonzalez said.

“Hysteria is starting to spread. The hospital is about to collapse. It’s at capacity,” he said, crying. “We need someone to help us immediately.”

The death toll from Maria in Puerto Rico was at least 10, including two police officers who drowned in floodwaters in the western town of Aguada. That number was expected to climb as officials from remote towns continued to check in with officials in San Juan.

Authorities in the town of Vega Alta on the north coast said they had been unable to reach an entire neighborhood called Fatima, and were particularly worried about residents of a nursing home.

“I need to get there today,” Mayor Oscar Santiago said. “Not tomorrow, today.”

Rossello said Maria would clearly cost more than the last major storm to wallop the island, Hurricane George in September 1998. “This is without a doubt the biggest catastrophe in modern history for Puerto Rico,” he said.

A dam upstream of the towns of Quebradillas and Isabela in northwest Puerto Rico was cracked but had not burst by Saturday afternoon as water continued to pour out of rain-swollen Lake Guajataca. Federal officials said Friday that 70,000 people, the number who live in the surrounding area, would have to be evacuated. But Javier Jimenez, mayor of the nearby town of San Sebastian, said he believed the number was far smaller.

Secretary of Public Affairs Ramon Rosario said about 300 families were in harm’s way.

The governor said there is “significant damage” to the dam and authorities believe it could give way at any moment. “We don’t know how long it’s going to hold. The integrity of the structure has been compromised in a significant way,” Rossello said.

The 345-yard dam, which was built around 1928, holds back a man-made lake covering about 2 square miles.

More than 15 inches of rain from Maria fell on the surrounding mountains, swelling the reservoir.

Officials said 1,360 of the island’s 1,600 cellphone towers were downed, and 85 percent of above-ground and underground phone and internet cables were knocked out. With roads blocked and phones dead, officials said, the situation may worsen.

“We haven’t seen the extent of the damage,” Rossello told reporters in the capital. Rossello couldn’t say when power might be restored.