(CNN) The US and British soldiers killed in an improvised explosive devise blast in Manbij, Syria, last week were on a classified mission to "kill or capture a known ISIS member" according to Pentagon spokesman Major Adrian Rankine-Galloway. The US military is releasing few details about the mission that killed Master Sgt. Jonathan J. Dunbar and UK soldier Sgt. Matt Tonroe.

They were killed and five other troops were wounded in a March 30 IED blast. The US military has not said if the IED was in a building, a vehicle or buried in the road. The troops were out of their vehicles at the time of the explosion, according to a US official. There is also no word on whether the target was found and captured or killed.

One indicator of the sensitivity of the mission is Dunbar was identified publicly by the Army only as being "assigned to Headquarters, U.S. Army Special Operations Command, Fort Bragg, N.C." That specific language has been used in the past when troops killed in action have actually been part of the Army's elite Delta Force, a counterterrorism unit that is not publicly identified.

Another US official confirmed to CNN that Dunbar was assigned to Delta. Delta Force and the Naval Special Warfare Development Group -- popularly known as Seal Team Six -- are among the so-called Tier One units that specialize in high-risk counterterrorism and hostage rescue missions that are rarely acknowledged. Special operations forces have been in the Manbij area recently looking for high-value ISIS operatives that may have fled to the area the US official said.

The US maintains about 2,000 US troops in Syria, who mostly work with local allies fighting ISIS in Syria.

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