The much-delayed launch of a new U.S. spy satellite has been postponed again due to an issue with its rocket, but it's unclear exactly when the mission will fly.

The United Launch Alliance, which built the Delta IV Heavy rocket that will launch the satellite, was aiming for a Jan. 6 launch from California's Vandenberg Air Force Base. But on Saturday (Jan. 5), ULA announced it was standing down from the launch try. The National Reconnaissance Office satellite, called NROL-71, was initially scheduled to launch in early December, ony to be delayed several times by technical issues and bad weather. The most recent launch attempt on Dec. 19 was called off due to a suspected hydrogen leak on the Delta IV Heavy booster.

"We continue to remedy the technical issues that caused the last scrub of the Delta IV Heavy, and are working with our partners, the National Reconnaissance Office and the U.S. Air Force, to ensure that we fly when it is safe to do so," Gary Wentz, ULA vice president of Government and Commercial programs, said in a statement. "We understand that this is a high-priority mission for the nation’s warfighters and we take our commitment to safety and mission assurance seriously."

The Delta IV Heavy rocket carrying the NROL-71 spy satellite for the U.S. National Reconnaissance Office is seen during tower rollback operations at Space Launch Complex-6 at Vandenberg Air Force Base in California. (Image credit: United Launch Alliance)

ULA initially attempted to launch the classifed NROL-71 mission on Dec. 7 and 8, but stood down due to technical issues. Bad weather postponed a Dec. 18 launch attempt, with the off-nominal hydrogen readings forcing the Dec. 19 scrub. ULA then announced a Dec. 30 launch target, before slipping to Jan. 6 ahead of the latest delay.

Email Tariq Malik at tmalik@space.com or follow him @tariqjmalik. Follow us @Spacedotcom and Facebook. Originally published on Space.com.