Western powers like the United States that would like to fight the extremists in Idlib are leery of endangering civilians and have invested heavily in local groups that oppose the jihadists.
The Syrian government and its allies, however, say Idlib is little more than a terrorist haven, where jihadists have imposed their control — a view some American officials share.
“Idlib Province is the largest Al Qaeda safe haven since 9/11,” Brett H. McGurk, the United States envoy to the coalition fighting the Islamic State, said last month. “Idlib now is a huge problem.”
Aid workers and residents say the situation there is more complicated, with a patchwork of groups struggling to provide necessary services to the civilians from all over Syria who have been bused to Idlib to live out the war. Though the extremist groups are militarily strong and the civilians have protested their presence, the militants have not systematically interfered with aid — at least not yet.
“Most people are thinking about the future, and they’re afraid of it,” said Nour Awwad, a media coordinator for Violet Organization, which works in Idlib. “But even if they’re afraid, where can they go?”
Much of Idilb, a poor, mostly rural province along the border with Turkey, joined the uprising against Mr. Assad in 2011, and armed rebel groups and Islamist militias soon formed.
For years, the United States and its allies sent covert aid to rebels, including many in the north, to fight Syrian government forces — a program that President Trump recently ended. Critics have charged that though the aid went to so-called moderate rebels, jihadists also benefited because they fought alongside the rebels and sometimes bought their weapons.
Elsewhere in Syria, the government was besieging opposition communities until they submitted, with the last rebels and civilians often bused to Idlib. This month, a few thousands refugees were sent from Lebanon to Idlib in a deal between Hezbollah, the Shiite militia that supports the Assad government, and the Levant Liberation Committee, the Al Qaeda affiliate formerly known as the Nusra Front.
The province’s population has swelled to two million, with nearly half those people displaced from elsewhere, the United Nations said.
That many needy people in one place has led to a large aid operation, with scores of groups sending food and other supplies across the border from Turkey, and establishing medical facilities and other projects in Idlib.
The future of those projects was thrown into doubt last month when clashes between the area’s armed groups left the Levant Liberation Committee as the dominant force. Though the group changed its name last year and said it had broken its ties with Al Qaeda, American officials dismissed the claim as propaganda.
They still consider the group to be a dangerous terrorist organization, as does Turkey, which has restricted the passage of commercial goods across its border, fearing that they would benefit the jihadists.
The border crossing where most of the province’s aid and commercial goods pass has long been a moneymaker for whoever controls it, and it appears that the jihadists are in a position to do so now.
Aid groups say they have not been forced to pay, presumably because the jihadists know that such demands would halt aid that people need. The jihadists have said they will create a civilian body to govern the province, but it remains unclear when that could occur or what such a body would look like.
Violence inside the province continues. On Saturday, seven members of the Syrian Civil Defense, a group also known as the White Helmets who dig people out of rubble after airstrikes, were shot dead in their office in Idlib by unknown attackers, the group said on Twitter.
The recent infighting between armed groups in Idlib has threatened the province’s economy, which residents said had been improving. Though no official cease-fire covers the area, it has been spared from Syrian and Russian airstrikes in recent months.
During the relative respite, business has taken off, with locals constructing new buildings and opening car dealerships and small factories. A Turkish aid group even opened a mall, where needy families shop with vouchers.
Some residents see Idlib as the last stand of the anti-Assad uprising and the start of a new Syrian society. Thousands of people in one town, Saraqib, participated in local elections last month, and the president of Idlib University was voted out of office recently — something the faculty noted had never happened to Syria’s president.
“It is like a phobia,” said Wissam Zarqa, an English teacher. “We don’t want anyone to stay as president for a long time.”
But as the rebels’ foreign backers, including the United States, have cut their support, the Assad government, backed by Russia and Iran, is expanding its control. At some point, most expect, the fight will come to Idlib.
“We are just going from one tragedy to a bigger tragedy,” said Muhammad Jaffa, an engineer who helps resettle displaced people. “They are sending everyone here and we don’t know what will happen to them in the end.”
Should new violence erupt, civilians have few options. Many fear arrest or conscription if they return to government areas, and Turkey has closed its border, where guards shoot — and sometimes kill — people sneaking across.
Ali al-Juma, a doctor who fled to Idlib from his home farther south, said he again feels trapped.
“It is now living in a minefield on the edge of another minefield,” he said.