ISTANBUL — A crushing military offensive by the Syrian government and its Russian allies in northern Syria has killed dozens of civilians and displaced more than 100,000 people in less than 10 days, humanitarian aid groups and medical officials there say.

The assault in Syria’s Idlib province is part of a push by President Bashar al-Assad to regain control of strategic highways, and ultimately the country’s last major rebel-held area. Clashes, government shelling and Russian airstrikes this month have sparked a panicked exodus that aid workers warn could lead to one of the worst humanitarian catastrophes of Syria’s eight-year civil war.

Homes in Maarat al-Numan, the largest city in Idlib’s southern countryside and the main target of the escalation, have steadily emptied as a parade of cars streams out, residents say. People are struggling to find medical care and shelter as the number fleeing airstrikes swells.

“People, I swear by God, are sleeping in open air under trees and the temperature at night is near freezing,” Shaker al-Humeido, a doctor who worked in Maarat al-Numan, said in a text message. The hospital where he worked had been emptied as fighting approached and he and his family fled north.

“I am shocked at the size of the tragedy,” he said.

Turkish President Recep Tayyip Erdogan warned last week that “massacres” in the province had sent more than 80,000 people fleeing toward Syria’s border with Turkey, which already hosts about 4 million Syrian war refugees. But his government, which maintains military outposts in Idlib and enjoys warm relations with Russia, has failed so far to blunt the offensive.

President Trump, writing on Twitter Thursday, said “Russia, Syria, and Iran are killing, or on their way to killing, thousands of innocent civilians in Idlib Province. Don’t do it! Turkey is working hard to stop this carnage.”

Iran is allied with Syria’s government and backs militias in Idlib, but does not have a significant presence there, and is not believed to be participating in the current offensive.

The violence is the latest miserable trial for Idlib, a wellspring of opposition to Assad’s government that hosts hundreds of thousands of people displaced by war from other parts of Syria.

The province, with 3 million people, has borne the brunt of a Russian and Syrian air campaign that has struck hospitals and leveled homes and markets, human rights groups say. The province and surrounding areas are largely controlled by Hayat Tahrir al-Sham, an extremist Islamist group that began as al-Qaeda’s affiliate in Syria and has tried to rebrand itself several times during the war.

Fighting over the past year has taken a disproportionate toll on children, according to UNICEF, which said in a statement Tuesday that more than 500 children were injured or killed in the first nine months of 2019. At least 65 children have been killed or injured in December alone, the group said.

Dareen Khalifa, a Syria analyst with the International Crisis Group, said Assad’s short-term goal has been to encircle and control Maarat al-Numan and the town of Saraqib, about 15 miles northeast. Then, she said, the Syrian army would push west to retake a highway linking Latakia and Aleppo as it attempted to capture Idlib in chunks.

“The problem is, the regime offensive that started in April hasn’t been very successful,” Khalifa said. “So now they are overcompensating by using devastating levels of air force. The casualties and displacement levels are catastrophic.

“If the regime continues and if the rebels don’t surrender, this will mean the worst humanitarian disaster we’ve seen in Syria.”

Naji Mustafa, a spokesman for the Turkish-backed rebel Syrian Liberation Front, said the government’s escalation “clearly aims at displacing people.”

“They are targeting marketplaces, hospitals, schools; they want the entire population of Maarat al-Numan, 80,000 people, to become displaced at the borders with Turkey. This has already started to happen.

“The clashes are severe,” he said. “We have lost some areas in the past few days, but we are pushing back to recapture them. This is how it has been.”

The previous offensive, launched in April, displaced 500,000 people in Idlib, according to aid groups.

“An additional half a million people could be displaced over the coming weeks if the violence continues to escalate,” said Kelly Razzouk, the U.N. director of the International Rescue Committee. “This would be the largest displacement seen since the war started eight years ago.”

Razzouk called reports of children living under olive trees in 30-degree weather “extremely distressing.”

“We are very concerned about the rates of malnutrition,” she said. “Eleven percent of children attending health clinics that we support are suffering from acute malnutrition, and food is being rationed. Nursing mothers are having to feed their infants herbal tea because they are malnourished and can’t feed their infants.”

Syrians activists have protested the offensive outside Russian missions around the world, including in Istanbul. There has also been growing criticism of Turkey’s government.

Mustafa Sejari, a senior official in the Turkey-backed Syrian National Army, published an open letter to Erdogan last week that highlighted the frustration.

“We understand the amount of internal and external pressure that you are under, but we, our people, our children, are getting killed,” Sejari wrote on Twitter. He asked Erdogan to take a “historic stand” by opening Turkey’s closed borders to women and children, and resuming military support to rebel forces.

Russian and Turkish officials discussed Idlib during a meeting Monday, Russia’s Foreign Ministry said in a statement. But Syrian activists say they’ve seen no visible improvements to the situation on the ground.

“A detailed exchange of views took place,” the Russians said.

Relief organizations warn they could soon be hampered in their ability to supply aid to Syrian civilians. China and Russia last week vetoed a U.N. Security Council resolution that would have renewed authorization for aid shipments across Turkey’s and Iraq’s borders for 12 months. The authorization expires Jan. 10.

“Given the scale of the needs in northern Syria, now is not the time to scale back,” said Razzouk, of the International Rescue Committee.

Families in the countryside around Maarat al-Numan have been fleeing their homes every night, said Tarek Mustafa, a physician at the city’s central hospital. The exodus from the city continues round-the-clock.

Medical staff had received injured front-line fighters and civilians, but also people who were in car accidents because they drove at night with their lights off, to avoid detection by warplanes.

Mustafa’s hospital was the last one functioning in the area, he said. On Sunday, it was strafed by gunfire from a helicopter, and by Monday, it had closed.