U.S. Defense Secretary James Mattis said on Monday that the United States will examine the proposed plan to create “de-escalation zones” to curb violence in the ongoing Syrian civil war between Bashar al-Assad’s regime, opposition rebels, and Islamic terrorist groups.

But, Reuters reported, Mattis said, “The devil’s always in the details” and it remains to be seen if the plan — brokered by Assad-backing Russia and Iran and rebel-backing Turkey — will work.

“All wars eventually come to an end and we’ve been looking for a long time how to bring this one to an end,” Mattis told reporters traveling with him to Copenhagen to meet with U.S. allies. “So we’ll look at the proposal and see if it can work.”

Mattis said that basic details remained unclear, including who specifically would ensure the zones were “safe” and exactly which groups would be kept out of them.

When reporters pressed Mattis about the plan he said: “The devil’s always in the details, right? So we’ve got to look at the details.”

The agreement proposes that four de-escalation zones be established in Syria for a period of six months, which could be extended if the three signatory countries agree to a continuation of the plan.

“The U.S. State Department has voiced concerns about the deal, saying it was skeptical of Iran’s involvement as a guarantor of the accord and Damascus’s track record on previous agreements,” Reuters reported.

Meanwhile, the U.S. continued its airstrikes against radical Islamic terrorists operating in Syria and Iraq.

“U.S. and coalition military forces continued to attack the Islamic State of Iraq and Syria yesterday, conducting 33 strikes consisting of 84 engagements,” Combined Joint Task Force Operation Inherent Resolve officials reported on Sunday.

“These strikes were conducted as part of Operation Inherent Resolve, the operation to destroy ISIS in Iraq and Syria. The destruction of ISIS targets in Iraq and Syria also further limits the group’s ability to project terror and conduct external operations throughout the region and the rest of the world,” task force officials said.

It is also unclear whether ISIS and other terrorists would comply with so-called safe zones in Syria. Last month, ISIS killed 33 people execution-style in eastern Syria, according to a human rights monitoring group.

“The Syrian Observatory for Human Rights reported that the terror organization carried out the mass killing in the the al-Mayadin desert near the strategic city of Deir Ezzor,” CNN reported.

The London-based monitoring group called it “the largest execution operation carried out by the Islamic State organization in 2017,” and said people between the ages of 18 and 25 were “killed by sharp tools.”

The report said it is unknown whether the victims were Syrian government forces, allied militia, or rebel factions.