Defense Secretary Mark Esper issued the US's strongest condemnation of Turkey's military assault on Kurdish forces in northeast Syria on Monday, calling it an "unacceptable incursion" and "impulsive."

"Due to Turkey's irresponsible actions, the risk to US forces in northeast Syria has reached an unacceptable level," Esper said. "We are also at risk of being engulfed in a broader conflict."

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Defense Secretary Mark Esper issued the US's strongest condemnation of Turkey's military assault against Kurdish forces in northeast Syria on Monday, calling it an "unacceptable incursion" and "impulsive."

In his statement, Esper characterized Turkey's assault as having "undermined the multinational 'Defeat ISIS' mission in Syria," and confirmed it resulted in the release of many of the ISIS fighters that were imprisoned by the once US-backed Kurdish forces.

Read more: Trump reportedly offered to sell F-35 jets to Turkey in exchange for not attacking Kurdish forces in Syria

Esper also noted concerns over consequences from Turkey's assault, which threatened US troops in the region on Friday. Turkish artillery fire nearly struck a small group US troops stationed near the Kurdish-held city of Kobani, according to multiple media reports. Although no US troops were injured in the shelling, the Defense Department said the artillery rounds came "within a few hundred meters" from their location.

"Due to Turkey's irresponsible actions, the risk to US forces in northeast Syria has reached an unacceptable level," Esper said. "We are also at risk of being engulfed in a broader conflict."

Turkish and US troops meet on the Turkish-Syrian border for a joint patrol in northern Syria, seen from near the Turkish town of Akcakale, September 8, 2019. Reuters

Attacks by Turkish forces and their partners in northeast Syria have so far killed numerous Kurdish fighters and civilians and come after President Donald Trump's decision to relocate US troops. Turkey has long cited the Kurdish People's Protection Units, or YPG, and the majority-Kurdish Syrian Democratic Forces as terrorist threats.

Critics of the decision say that many ISIS militants who were detained by the Kurdish fighters could go free, as the Kurds shift their forces to address the Turkish threat. On Monday, Trump suggested without evidence that the Kurds "may be releasing" ISIS prisoners to drag the US into the conflict and that he based his decision on a desire to avoid further wars overseas.

On Sunday, the UN estimated that more than 130,000 people were displaced as a result of the conflict. The UN also estimated that up to 400,000 people in the region may require aid and protection in the near future.

"Turkey's unilateral action was unnecessary and impulsive," Esper said, adding that US-Turkish relations were now "damaged."

"President Erdogan bears full responsibility for its consequences, to include a potential ISIS resurgence, possible war crimes, and a growing humanitarian crisis," he added.