WASHINGTON (AP) — Trucks carrying badly needed aid for the Gaza Strip rolled across a newly built U.S. pier and into the besieged enclave for the first time Friday as Israeli restrictions on border crossings and heavy fighting hindered the delivery of food and other supplies.

The shipment is the first in an operation that American military officials anticipate could scale up to 150 truckloads a day, all while Israel presses in on the southern city of Rafah in its seven-month offensive against Hamas. At the White House, National Security Council spokesman John Kirby said “more than 300 pallets” of aid were in the initial delivery and handed over to the U.N., which was preparing it for distribution.

Kirby said the U.S. has gotten indications that “some of that aid was already moving into Gaza.”

But the U.S. and aid groups warn that the floating pier project is not a substitute for land deliveries that could bring in all the food, water and fuel needed in Gaza. Before the war, more than 500 truckloads entered the territory on an average day.

The operation’s success also remains tenuous because of the risk of militant attack, logistical hurdles and a growing shortage of fuel for the aid trucks due to the Israeli blockade of Gaza since Hamas’ Oct. 7 attack. Militants killed 1,200 people and took 250 others hostage in that assault on southern Israel. The Israeli offensive since has killed more than 35,000 Palestinians in Gaza, local health officials say, while hundreds more have been killed in the West Bank.

Aid agencies say they are running out of food in southern Gaza and fuel is dwindling, while the U.S. Agency for International Development and the U.N. World Food Program say famine has already taken hold in Gaza’s north.

Troops finished installing the floating pier on Thursday, and the U.S. military’s Central Command said the first aid crossed into Gaza at 9 a.m. Friday. It said no American troops went ashore in the operation.

The Pentagon said no backups were expected in the distribution process. The U.S. plan is for the United Nations, through its World Food Program, to take charge of the aid once it leaves the pier. This will involve coordinating the arrival of empty trucks and their registration, overseeing the transfer of goods coming through the floating dock to the trucks and their dispatch to warehouses across Gaza, and, finally, handing over the supplies to aid groups for delivery.

The U.K. said some of its aid for Gaza was in the first shipment that went ashore, including the first of 8,400 kits to provide temporary shelter made of plastic sheeting. And it said more aid, including 2,000 additional shelter kits, 900 tents, five forklift trucks and 9,200 hygiene kits, will follow in the coming weeks.

“This is the culmination of a Herculean joint international effort,” said Prime Minister Rishi Sunak. “We know the maritime route is not the only answer. We need to see more land routes open, including via the Rafah crossing, to ensure much more aid gets safely to civilians in desperate need of help.”

Aid distribution had not yet begun as of Friday afternoon, said a U.N. official who spoke on the condition of anonymity because of the sensitivity of the matter. The official said the process of unloading and reloading cargo was still ongoing.

The U.N. humanitarian aid coordinating agency said the start of the operation was welcome but not a replacement for deliveries by land.

“I think everyone in the operation has said it: Any and all aid into Gaza is welcome by any route,” Jens Laerke, spokesman of the Office for the Coordination of Humanitarian Affairs, told journalists in Geneva on Friday. Getting aid to people in Gaza “cannot and should not depend on a floating dock far from where needs are most acute.”

The U.N. earlier said fuel deliveries brought through land routes have all but stopped and that would make it extremely difficult to bring the aid to Gaza’s people.