"This operation was part of the Coalition's mission to defeat ISIS, and we remain focused on our mission," Rankine-Galloway said.
The kill or capture mission was first reported by CNN. In earlier press releases, the Department of Defense characterized the incident that killed Dunbar and Tonroe as occurring while the group was on "patrol."
Dunbar, 36, of Austin, Texas, was assigned to the Army's elite Delta Force, according to two sources familiar with his service. He joined the Army in 2005, serving multiple tours in both Afghanistan and Iraq, and is only the second American combat-related death this year.
Tonroe, 33, was a member of a British special forces unit, Special Air Service.
In Manbij, the U.S. and its coalition partners have a small contingent of forces advising, training and assisting Syrian Democratic Forces in the fight against ISIS.
But the death of these soldiers comes as the White House has placed doubt on the future role for the U.S. in Syria.
"By the way, we're knocking the hell out of ISIS," Trump told a crowd in Ohio during a speech on infrastructure spending. "We're coming out of Syria very soon. Let the other people take care of it now, very soon. Very soon, we're coming out."
"CENTCOM's military strategy in Syria is the defeat of ISIS," a spokesperson said in an email. "To reiterate what [CENTCOM commander Gen. Joseph Votel] said yesterday, our mission in northern Syria and Syria in the coalition is strictly focused on the defeat of ISIS so as we work through the very complicated situation in Syria, it is our intention to continue to focus on the aspects of ISIS that still need to be addressed."
Trump's comment also contradicted how the president himself has spoken about American military action – as he has repeatedly insisted the U.S. not set timelines or telegraph actions to the enemy.
Mattis has made the same argument, writing in a letter to Congress in January, "We do not have a timeline-based approach to our presence in either Iraq or Syria."
Withdrawing "prematurely," he added, would only give ISIS the opportunity "to regenerate capabilities and reestablish local control of territory... We, along with the Coalition and our partners, remain committed to ISIS's permanent defeat."
While the U.S.-led coalition has made significant progress against ISIS, the group has not been destroyed completely. Meanwhile, Syrian dictator Bashar al-Assad continues to wage a civil war on his own people – backed by the Russian government.