Fuel needed to power the two bakeries was delivered by the United Nations last Sunday and was scheduled to run out by Friday, Abeer Etefa, a spokeswoman for the U.N. World Food Program, said. It was not clear when more fuel would arrive, she said.

Northern Gaza has been largely cut off from aid since Israel’s military offensive in Gaza began in October, and most bakeries there have been closed for months because of the fighting.

Last week, a senior U.S. official told Congress that expert projections showed the north was experiencing a famine, but on Wednesday Prime Minister Benjamin Netanyahu of Israel rejected that claim, saying “Israel was going above and beyond in the humanitarian sphere.”

Facing pressure from the Biden administration and other allies to ease the humanitarian crisis, Israel has been eager to show that more aid is entering northern Gaza, especially since its strike on April 1 that killed seven aid workers with World Central Kitchen, a food charity.

This week, the United Nations shared video online showed bags of flour piled high in bakery storerooms, and Palestinian children clapping for the aid truck from their windows.

The two bakeries were able to reopen because the United Nations, with the permission of the Israeli military, was able to bring in enough fuel and flour in recent days. Still, the World Food Program said that cooperation needs to continue to avert a disaster. “We need safe and sustained access to prevent famine,” the organization said.

Matt Miller, a spokesman for the U.S. State Department, praised the bakery openings but said more needed to be done.

“We have seen improvement — not yet to the level where it needs to be, and certainly once it gets to that level, we need to see it sustained over time, and that’s how we’re going to judge things,” he told reporters on Tuesday.

Kamel Ajour, who owns one of the bakeries, said that the two bakeries together can turn out a million pieces of bread each day. The U.N. provided the fuel and flour to run both operations, he said.

“Bread is the most important thing in life after water,” Mr. Ajour, 51, said in a phone interview. “The starvation in northern Gaza is very dangerous. We need to put an end to it. This effort is one of the solutions.”

Bags of 50 pieces of bread were sold for five shekels, or roughly $1.30, he said. The U.N. said that was the lowest price bread has sold for in months.

But Mazen Harazeen, 39, a paramedic in Gaza City with nine children, said even five shekels was too much for many Gazans. He has continued to work as a paramedic during the war, but has not received a full paycheck in six months.

“People line up there for around three hours to get one and only one bag of bread,” Mr. Harazeen said, adding that he had walked nearly two miles to reach one of the bakeries. Their impact, he said, would be “very small.”

Maher Al-Mashharawi, 28, a software developer in Gaza City, said he had tried several times to get bread from one of the two bakeries, but on every attempt was stymied by a seemingly endless line and left empty-handed.

“It’s really frustrating,” he said. “We’ve been hoping things will get better, but we’re still facing difficulties.”

But he said food prices had dropped significantly in recent days. A 25-kilogram bag of flour now costs 70 shekels compared to 1,900 during the worst days of the war, he said, and a kilogram of apples was 20 shekels, down from 120.

Israel’s bombing campaign and ground invasion have devastated the north, leading to a breakdown in civil order and chaos and violence engulfing many efforts to distribute aid.

Mr. Ajour said security for the bakeries was being provided by “private companies” and by bakery workers themselves, and that fences were built around both operations. He did not elaborate on who ran or staffed the private security companies.

Abu Bakr Bashir contributed reporting.