Air strikes in the rebel-controlled Idlib province of Syria are raising fears that the Syrian government will reclaim the territory at an enormous cost to civilians’ lives.

An offensive on Idlib would mark “the beginning of the end of the Syrian civil war,” according to North Dakota State political science professor Thomas Ambrosio. Wedged up against the Turkish border in the northwest corner of Syria, Idlib is one of the last major areas of Syria not under government control. President Bashar al-Assad’s ally Russia began shelling Idlib on Tuesday, Sept. 4. Different sources report varying counts on how many civilians were killed.

Ambrosio expects the attacks will accelerate significantly in the next week. He also said the impending military advances will be the deadliest since the war began in 2011.

“This is going to be a horrible humanitarian disaster,” Ambrosio said. According to the Syrian Network for Human Rights, more than 217,000 civilians have been killed in Syria since 2011. That number is expected to rise if President Assad and his allies press into Idlib to quell the rebel forces.

Until now, Idlib was a relatively safe haven for civilians fleeing from other parts of the country. The Atlantic reports that more than 3 million civilians are living in Idlib. One-third of those 3 million people are refugees. Since Turkey has closed its border with Syria, an offensive launched on Idlib by Assad and his allies would mean that the people living there would have nowhere to go.

“I think by all accounts, the regime is going to stop at nothing to take this territory back,” Mona Yacoubian, an expert on Syria at the U.S. Institute of Peace, told the Atlantic. “And so we expect a particularly brutal onslaught on the part of the Syrian regime backed by the Russians.”

U.S. President Donald Trump warned Assad, Russia and Iran not to “recklessly attack Idlib Province” in a tweet Monday, Sept. 3. He wrote that the death toll could be in the hundreds of thousands. Turkey, Iran and Russia met at a summit in Tehran on Friday to discuss the next steps. In an article by the Guardian, the Turkish government described the summit as “the last chance to avoid a massacre.”

Ambrosio said he thinks eventually Turkey will have to reopen its border to refugees fleeing the chaos.

If Assad’s regime successfully retakes Idlib, Ambrosio said he expects a settlement will be reached quickly. Still, Ambrosio holds little hope that the days after the war will be any less grim.

“The Syrian Civil War is a symptom of other changes going on both in the Middle East and internationally,” Ambrosio said. “We should expect to see more Syria’s moving forward, not less.”