CAIRO » In one of Yemen’s largest cities, residents have been reduced to a diet of rice. In another, they sleep overnight in lines waiting for gasoline. Hospitals may soon be forced to close, bombs and shells are raining down ever more thickly and randomly, and in places snipers target anyone brave or desperate enough to walk the streets.

Five weeks after the start of a Saudi-led bombing campaign, more than 1,000 Yemenis have died in the fighting and the United Nations says that at least 300,000 people have been displaced, forced to hunt for food and fuel in a country bereft of both. Fierce fighting in the port city of Aden has killed dozens of people in the past few days, left neighborhoods in flames and set off a panicked exodus.

On Thursday, little more than a week after Saudi Arabia said it was shifting the focus of its military operations in Yemen to make way for political negotiations and humanitarian relief, residents of the capital of Sanaa were subjected to some of the heaviest airstrikes since the bombing began.

The worsening humanitarian crisis, a deepening civil war that threatens to inflame the regional struggle between Saudi Arabia and Iran and little evidence that either side is winning or seriously pursuing negotiations, have raised stark questions about both the strategic goals of the Saudi-led military campaign and the international response.

"It’s insanity — here are 25 million people in the middle of this conflict," said Nuha Abdulljabbar, a humanitarian coordinator for Oxfam, the British charity. The combatants, she added, are "condemning the whole country to die."

Indeed, this week, coalition warplanes bombed at least two airports to prevent an Iranian plane from landing; the strikes damaged a runway that was being used for international aid deliveries. And the fighting, as well as a naval cordon set up by the coalition, is blocking deliveries of fuel and food by sea to a country that depends critically on imports.

Under heavy international pressure, including from the United States, the Saudis announced last week that they were ending the first phase of their military campaign against the Houthis, the northern Yemeni rebel group that has captured large parts of the country over the last eight months, including Sanaa.

But instead of scaling back the airstrikes, the coalition has broadened them, demonstrating what appears to be a determination to defeat the Houthis, rather than any inclination to negotiate. with the group, which is allied with Iran, Saudi Arabia’s regional nemesis. They have also started more aggressively supplying weapons to the Houthis’ adversaries, according to fighters in Aden and Taiz, even though those weapons have frequently ended up in the hands of either the Houthis or profiteers.

Saudi Arabia had threatened a ground invasion of Yemen, but faced difficulties convincing allies to provide troops for such an operation. Instead, analysts said, the Saudis were likely to intensify their efforts to support proxy forces in Yemen, a strategy they have used before.

A Reuters article Wednesday quoted a Yemeni official as saying the Saudis had trained hundreds of Yemeni tribesmen to fight the Houthis, though Saudi officials did not confirm the report.

For their part, the Houthis and their allies — forces loyal to Ali Abdullah Saleh, the former president — have also brushed off negotiations in favor of expanding their bloody offensive aimed at controlling strategic cities.

And the government of President Abed Rabbo Mansour Hadi, whom the Saudis had promised to restore to power, remains outside Yemen, trying to influence events from hundreds of miles away in the Saudi capital, Riyadh.

While the fighting rages, it is the civilians who suffer. On Tuesday, the Saudi-led coalition struck near a military base in Sanaa that residents said had been bombed dozens of times over the last month.

Airstrikes in the area on April 20 set off a huge explosion that flattened much of a nearby residential area, killing at least 36 people and wounding hundreds of others. Residents picking through the debris were still in shock days later, after the bombing had been eclipsed by more battles and bloodshed.

Hussein Mahdi Howban, a 25-year-old student, said the blast had blown out the doors and windows of his apartment and sent him flying through the air. Covered in blood and dust, he stumbled outside to find that the explosion had brought buildings and utility poles down on top of families. "I saw my neighbors crying, screaming," he said.

The Houthis’ attempts to capture Aden and Taiz, Yemen’s third-largest city, have taken a crushing toll on residents. Witnesses said the Houthis had all but demolished the Khormaksar neighborhood in central Aden over the last few days, ordering locals to leave and then blowing up houses with tank fire. Houthi fighters also occupied a hospital in the neighborhood as patients and doctors fled, according to residents and aid workers.

As sections of Aden were reduced to rubble, supplies dwindled for survivors. "Lunch is rice, dinner is rice," said Faris al-Shuabi, a Khormaksar resident. "Rice is the main meal."

In Taiz, northwest of Aden, the Houthi and Saleh forces have been accused of using similarly indiscriminate force, hitting at least two hospitals during clashes. The number of Yemenis fleeing the combat is sharply increasing, the United Nations said this week. Its estimate of 300,000 people displaced within the country doubled in just 11 days.

Yemenis on the run are finding fewer and fewer places untouched by the war. The shortages of fuel and food are crippling Taiz and Aden, as well as Sanaa and other northern areas.

Shipments to Yemen have been blocked by the fighting at its ports, including Aden, and by the naval cordon imposed by the Saudi-led coalition, which inspects all shipments, according to shipping industry analysts.

Several fuel tankers are believed to be offshore, waiting to dock in Hodeidah. It was unclear if the coalition was stopping them or if they were standing off for other reasons, including their safety, the analysts said.

In Sanaa, people seeking fuel are waiting in line for days. "People sleep at the gas stations," said Marie Claire Feghali, a spokeswoman for the International Committee of the Red Cross who is based in Sanaa.

The fuel shortage has set off a "vicious circle" of suffering, she said. Yemen, which imports 90 percent of its food, has received hardly any food shipments in the past five weeks, and without fuel, there is no way to transport what little food is available.

In a nation already severely short of water, water pumps depend on fuel to operate. So do power generators, and the cold storage areas for medicines and vaccinations in hospitals.

The Yemeni authorities said this week that they might have to shut down the telecommunications network as well, further isolating a population that is increasingly cut off from the world.

One city seemed to be escaping the worst of the crisis: the southern port of Al Mukalla, which has been controlled by the local al-Qaida branch since early April.

The militants have focused on security, and the city has been administered by a council of residents and tribal leaders under their supervision. Though fuel is in short supply, there have been no lines at bakeries, and the shops are still stocked with food, said Faris bin Hilabi, a local businessman.

Dairy products have been arriving overland from Oman, and other essentials have come from Saudi Arabia, Hilabi said. The coalition authorities have allowed at least one ship to dock in the port with a load of sugar, and a shipment of wheat from Ukraine is on the way.

Sanaa has not been as lucky. On Thursday, after workers had repaired the runway at the international airport so that flights could resume, the Saudi-led coalition bombed the airport again.

Kareem Fahim, New York Times