Mobile kitchen units, truckloads of tools and massive quantities of supplies are ready to help people affected by Hurricane Harvey, but there is one problem: Much of the material is hundreds of miles away from Houston.

The extent of the disaster caused by Hurricane Harvey and its remnants has made it impossible for relief agencies to help in some of the hardest-hit areas. The storm lashed hundreds of miles of coastline from Corpus Christi, Tex., to Lake Charles, La., flooded more than one-quarter of Harris County, Tex., home to the city of Houston and more than 4.5 million people, and inundated the Beaumont-Port Arthur area with more than two feet of rain in 24 hours. Trucks now sit hundreds of miles away, waiting for roads to become passable and for search-and-rescue teams to finish their jobs.

“The scope and size of the storm is unlike anything the [Salvation Army] has ever experienced,” said Lt. Col. Ronnie Raymer.

Trying to navigate flooded roads and an ever-changing situation as levees fail, rivers overflow and tides change makes it too difficult, even dangerous to try getting supplies in. The biggest issue is that the storm just won’t let up: Tuesday and Wednesday, the Beaumont-Port Arthur area was heavily flooded and rains drenched western Louisiana.

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“We’ve got units set up as close as we can,” said Mike Ebert, a spokesman for the North American Mission Board of the Southern Baptist Convention. Many of the emergency units are in Mississippi. The mission board has deployed kitchens traveling to smaller towns, including Rockport on the Texas coast, to feed first responders. The units also contain showers and laundry facilities.

Some relief organizations cannot get supplies into Houston, where highways are flooded and authorities are still searching for and rescuing people from homes. Organizations that do not help with basic shelter needs will wait until the situation changes from search and rescue to a recovery mission to help people whose homes are flooded.

“What we don’t want to do right now is get our volunteers in there and have them get stranded and need to get rescued and create more problems for first responders,” Ebert said.

Tim Haas, manager of disaster relief at Samaritan’s Purse, said the organization has more than 60 volunteers in Victoria and another base of operations in the Portland-Rockport area. Three other disaster relief units — 53-foot trailers filled with tools — will be stationed in Baton Rouge and ready to deploy when it is possible to safely access hard-hit areas. Samaritan’s Purse also has generators and mobile kitchens in Baton Rouge, where it has been active since massive floods there last year. It is hoping to get a unit to a church in Santa Fe, Tex., between Houston and Galveston.

But the flooding makes meeting even immediate needs difficult to fill. A mobile kitchen capable of feeding 40,000 people a day was expected to arrive at the George R. Brown Convention Center in Houston on Tuesday but could not get there until Wednesday, according to the Red Cross.

“It can be frustrating as people are wondering: When can we get deployed?” said Bill Bumpas, a spokesman for the Southern Baptists of Texas Convention Disaster Relief. Some, however, do not have to go far: One man waiting to be deployed has rescued people from rising water in his own neighborhood, Bumpas said.

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The magnitude of Harvey has shocked even the most seasoned disaster workers. Because the storm devastated so many places in different ways, with varying combinations of high winds and flooding, the response will be different in each area. And because of the storm’s massive geographic reach and impact on the highways and roads, organizations are realizing that setting up one centralized command center will not be effective.

“We also quickly realized this is going to stretch us more than Katrina did and that we’re going to have to begin thinking outside the box in how we move forward, especially in long-term recovery,” Raymer said.

Raymer said the Salvation Army is starting to think about opening centers in different neighborhoods because Houston’s sprawl was difficult to navigate even before the storm. The Salvation Army has opened shelters in the city and a command center in northwest Houston. On Wednesday, Raymer was in Rockport, which took a direct hit from Harvey when it was at hurricane strength. Raymer said that Rockport is “devastated” and that he did not see one undamaged building.

“There are multiple levels of challenges just dealing with how you navigate Houston, how you get around town,” said Haas, of Samaritan’s Purse. But conditions are improving ever so slightly.

“The island is expanding” as some floodwaters recede, said Charles Maltbie, a Red Cross spokesman. The organization has not been able to move resources assembled in Austin to Houston because of flooded roads.

But some on the ground in Houston would prefer to understand the scope of the disaster before assistance starts arriving in large volumes.

Anna M. Babin, president and chief executive of the United Way of Greater Houston, said the organization’s 211 help line has had a backlog of 9,000 calls. Because the storm is still unfolding nearly a week after its initial landfall, she is urging organizations to hold off on sending materials.

“People keep wanting to send us stuff and bring in trucks, and we have to say, ‘Wait. Where are we going to put these?’ ” she said. “We’re still in the disaster.”