Kurdish militia leaders in Syria called the U.S. withdrawal announced Sunday in the region a “stab in the back” and vowed to defend their positions against Turkey “at all costs.”

The United Nations expressed grave concerns about civilian casualties from the Turkish offensive, while European leaders urged Turkish President Recep Tayyip Erdogan to reconsider his plans.

The White House issued a statement on Sunday after President Donald Trump spoke with Erdogan by phone, indicating the U.S. military “will not support or be involved in” Turkey’s operation.

“United States forces, having defeated the ISIS territorial ‘Caliphate,’ will no longer be in the immediate area,” the White House said.

The statement indicated U.S. dissatisfaction with European contributions to resolving the problem of captured Islamic State fighters held in Syria and indicated those prisoners are now Turkey’s problem:

The United States Government has pressed France, Germany, and other European nations, from which many captured ISIS fighters came, to take them back, but they did not want them and refused. The United States will not hold them for what could be many years and great cost to the United States taxpayer. Turkey will now be responsible for all ISIS fighters in the area captured over the past two years in the wake of the defeat of the territorial “Caliphate” by the United States.

A spokesman for President Erdogan said on Monday that Turkey aims to create a “safe zone” along the Turkish-Syrian border free of “terrorist elements” into which refugees from Syria can be safely returned. During their phone call on Sunday, Erdogan expressed “frustration” to Trump over “the U.S. military and security bureaucracy’s failure” to fulfill an August arrangement for creating such a buffer zone.

The U.S. has been attempting to delay the proposed Turkish incursion, which would target Kurdish militia known as the People’s Protection Units (known by the Kurdish acronym YPG and YPJ, for the female forces) considered by Erdogan to be terrorist allies of Kurdish separatists belonging to the PKK party in Turkey. The YPG was a U.S. battlefield ally against the Islamic State during the Syrian civil war. The Kurdistan Workers’ Party (PKK) is a U.S.-designated Marxist terrorist group.

Kurdish commanders also lead an umbrella group known as the Syrian Democratic Forces (SDF), which on Monday denounced the White House announcement as a “stab in the back” that would have a “great negative” impact on the ongoing war against ISIS.

A senior SDF official named Mustafa Bali told the Kurdish Rudaw news service the United States was “leaving the area to turn into a war zone.”

“The SDF is determined to defend NE Syria at all costs,” Bali said.

“U.S. forces on the ground showed us that this is not how they value friendship and alliance,” Bali said on Twitter. “However, the decision by [President Trump] is about to ruin the trust and cooperation between the SDF and US built during the fight against ISIS. Alliances are built on mutual trust.”

“We are not expecting the US to protect northeastern Syria. But people here are owed an explanation regarding security mechanism deal, destruction of fortifications and failure of US to fulfill their commitments,” he added.

Bali told Voice of America News over the weekend that the SDF sees Turkey’s military threats as “extremely serious” and fears “mass killings would be committed against our people if Turkish forces invaded this part of Syria.”

The SDF issued a statement repeating Bali’s comments and urging all “Kurdish, Arab, Assyrian, and Syriac people to strengthen their unity and stand by the SDF in defense of their land” against the Turks.

A spokesman for President Erdogan responded by insisting Turkey will act only to eliminate members of the YPG “terrorist group” inside a small buffer zone and insisting Ankara “supports Syria’s territorial integrity and political unity.”

The statement from Erdogan’s office insisted Turkey “has no interest in occupation or changing demographics” and accused the PKK and YPG of already making efforts to do so.

“Turkey fights against a terrorist organization that has also killed and oppressed the Kurds,” the statement declared.

A White House official told Reuters that U.S. troops are initially moving out of the “safe zone” area near the Turkish border, not the entire Syrian theater. Another official said the U.S. still considers the planned Turkish invasion a “bad idea” and said the U.S. “will not help it in any way, but will also not resist it.”

A Pentagon spokesman told VOA News that an “uncoordinated military operation by Turkey” could “undermine our shared interest of a secure northeast Syria and the enduring defeat of ISIS.”

The United Nations on Monday expressed concerns about civilian casualties from the Turkish operation and worried it could trigger a new wave of refugees.

“Any operation that takes place at the moment has to take into account to ensure that we don’t see any further displacement. We are hoping for the best but preparing for the worst,” said U.N. regional humanitarian coordinator Panos Moumtzis on Monday.

Mountzis said the U.N. has a “bitter history” with the concept of safe zones and “we never promote or encourage” such plans.

“We don’t think it is something that had worked for the United Nations, keeping in mind Srebrenica and what had happened in the past,” he said, referring to the 1995 slaughter of some 8,000 Muslims by Bosnian Serb troops in a U.N. safe zone.

“Our hope is that there will be full cooperation by all to make sure that it happens as smoothly as possible, without resulting in displacement, and ensuring protection of civilians, ensuring that the basic principles of humanity will be respected on the ground,” Mountzis said of the impending Turkish operation.

The European Union on Monday recognized “Turkey’s legitimate concerns” but restated its position that “any sustainable situation will not be reached by military means.” The French Foreign Ministry warned Turkey it could “hurt regional stability” and trigger a new refugee crisis by invading Syria.

Turkey is a member of the North Atlantic Treaty Organization (NATO). Although NATO itself was quiet Sunday and Monday about the planned Turkish operation, U.S. Senators Lindsey Graham (R-SC) and Chris Van Hollen (D-MD) introduced a bill on Monday that would level sanctions against Turkey and demand its suspension from NATO if the invasion proceeds.