AFAR, Ethiopia — The road, a 300-mile strip of tarmac that passes through some of the most inhospitable terrain on earth, is the only way into a conflict-torn region where millions of Ethiopians face the threat of mass starvation.

But it is a fragile lifeline, fraught with dangers that have made the route barely passable for aid convoys trying to get humanitarian supplies into the Tigray region, where local fighters have been battling the Ethiopian army for eight months.  Now the road is barely passable, making aid delivery from the United Nations difficult.CreditCredit…Finbarr O’Reilly for The New York Times

Aid workers say the main obstacle is an unofficial Ethiopian government blockade, enforced using tactics of obstruction and intimidation, that has effectively cut off the road and exacerbated what some call the world’s worst humanitarian crisis in a decade.

In the past month, just a single United Nations aid convoy of 50 trucks has managed to travel this route. The U.N. says it needs twice as many trucks, traveling every day, to stave off catastrophic shortages of food and medicine inside Tigray.

Yet nothing is moving.

On Tuesday, the World Food Program said 170 trucks loaded with relief aid were stranded in Semera, the capital of the neighboring Afar region, waiting for Ethiopian permission to make the desert journey into Tigray.

“These trucks must be allowed to move NOW,” the agency’s director David Beasley wrote on Twitter. “People are starving.”

The crisis comes against the backdrop of an intensifying war that is spilling out of Tigray into other regions, deepening ethnic tensions and stoking fears that Ethiopia, Africa’s second most populous nation, is tearing itself apart.

Inside Tigray, the needs are dire, and rapidly rising. The United Nations estimates that 400,000 people there are living in famine-like conditions, and another 4.8 million need urgent help.

Ethiopian and allied Eritrean soldiers have stolen grain, burned crops and destroyed agricultural tools, according to both aid groups and local witnesses interviewed by The New York Times. This has caused many farmers to miss the planting season, setting in motion a food crisis that is expected to peak when harvests fail in September.

The Ethiopian prime minister, Abiy Ahmed, who won the 2019 Nobel Peace Prize, said last week that his government was providing “unfettered humanitarian access” and committed to “the safe delivery of critical supplies to its people in the Tigray region.”

But Mr. Abiy’s ministers have publicly accused aid workers of helping and even arming the Tigrayan fighters, drawing a robust denial from one U.N. agency. And senior aid officials, speaking on the condition of anonymity to avoid jeopardizing their operations, said the government’s stated commitment to enable aid deliveries was belied by its actions on the ground.

Aid workers have been harassed at airports or, in the case of a World Food Program official last weekend, have died inside Tigray for want of immediate medical care.

Billene Seyoum Woldeyes, a spokeswoman for Mr. Abiy, said federal forces had left behind 44,000 tons of wheat and 2.5 million liters of edible oil as they withdrew from Tigray in June. Any hurdles to humanitarian access were being “closely monitored” by the government, she said.

But on the ground, vital supplies are rapidly running out — not just food and medicine, but also the fuel and cash needed to distribute emergency aid. Many aid agencies have begun to scale back their operations in Tigray, citing the impossible working conditions. Mr. Beasley said the World Food Program would start to run out of food on Friday.

Fighting is raging along what had once been the main highway into Tigray, forcing aid groups to turn to the only alternative: the remote road connecting Tigray to Afar that runs across a stark landscape of burning temperatures.

When I traveled the route on July 4, the war in Tigray had just dramatically reversed direction.

Days earlier, Tigrayan fighters had marched into the regional capital, Mekelle, hours after beleaguered Ethiopian soldiers quit the city. The city airport was shut, so the only way out of Tigray was on a slow-moving U.N. convoy that took the same desolate route out as the fleeing Ethiopian soldiers.

We drove down a rocky escarpment on a road scarred by tank tracks. As we descended into the plains of Afar, the temperature quickly rose.

Our minivan raced across a barren field of dried lava that stretched for miles. Sand drifted onto the road in places, and the van’s roof grew too hot to touch.

Our driver chewed leaves of the mild narcotic khat as he gripped the wheel, frequently steering us onto the wrong side of the road. It didn’t matter — the only vehicles we passed were broken-down trucks, their sweating drivers poring over greasy entrails.

In the handful of villages we crossed through, people sheltered from the sun inside buildings covered with tin sheets and heavy blankets. My weather app said it was 115 degrees outside. Then my phone issued a text warning that it was overheating.

We passed 13 checkpoints, the initial ones manned by militia fighters and then later ones guarded by Ethiopian government forces. We reached Semera after 12 hours.

Days later, a second U.N. convoy headed out of Tigray was not so lucky.

According to an aid worker on the convoy, Ethiopian federal police subjected Western aid workers to extensive searches along the way, and later detained seven Tigrayan drivers overnight after impounding their vehicles. The drivers and vehicles were released after two days.

On July 18, a 10-vehicle U.N. convoy carrying food to Tigray came under attack 60 miles north of Semera when unidentified gunmen opened fire and looted several trucks, according to the World Food Program. The convoy turned around, and all aid deliveries along the route have since been suspended.

In a statement, Mr. Abiy’s office blamed the attack on the Tigray People’s Liberation Front, the former ruling party of the Tigray region that the national government’s forces have been fighting.

But two senior U.N. officials, speaking on the condition of anonymity to avoid worsening relations with the Ethiopian authorities, said they believed the attack had been carried out by a pro-government militia at the behest of the Ethiopian security forces.

A rare humanitarian flight to Tigray four days later confirmed fears among aid workers that the Ethiopian authorities were pursuing a strategy of officially permitting humanitarian access while in practice working to frustrate it.

At the main airport in Addis Ababa, 30 aid workers boarding the first U.N. flight to Mekelle in more than a month were subjected to intensive searches and harassment, several people on board said. Ethiopian officials prohibited aid workers from carrying cash greater than the equivalent of $250, satellite phones and personal medication — the last restriction resulted in an official with Doctors Without Borders having to get off the flight. Six hours late, the flight took off.

The World Food Program publicized the flight but made no mention of the delays or harassment — an omission that privately angered several U.N. officials and other aid workers who said it followed a pattern of U.N. agencies being unwilling to publicly criticize the Ethiopian authorities.

Further complicating the aid effort: The war is now spilling into Afar.

In the past week Tigrayan forces have pushed into the region. In response Mr. Abiy mobilized ethnic militias from other regions to counter the offensive.

Mr. Abiy has also resorted to increasingly inflammatory language — referring to Tigrayan leaders as “cancer” and “weeds” in need of removal — that foreign officials view as a possible tinder for a new wave of ethnic violence across the country.

Ms. Billene, his spokeswoman, dismissed those fears as “alarmist.” The Ethiopian leader had “clearly been referring to a terrorist organization and not the people of Tigray,” she said.

Inside Tigray, the most pressing priority is to reopen the road to Afar.

“This is a desperate, desperate situation,” said Lorraine Sweeney of Support Africa Foundation, a charity that shelters about 100 pregnant women displaced by fighting in the Tigrayan city of Adigrat.

Ms. Sweeney, who is based in Ireland, said she had fielded calls from panicked staff members appealing for help to feed the women, all of whom are at least eight months pregnant.

“It brings me back to famine times in Ireland,” Ms. Sweeney said. “This is crazy stuff in this day and age.”