Washington (AFP) - A Virginia school bus ferrying students to and from classes this week had explosives under the hood after the CIA accidentally left them following a training exercise, the intelligence agency said.

"Last week, a CIA K-9 unit conducted a routine training exercise with law enforcement officials in Loudoun County, VA," the CIA said in a statement on Thursday. "During the exercise, explosive training material was inadvertently left by the CIA K-9 unit in one of the buses used in the exercise."

The bus was used Monday and Tuesday to transport high school and elementary students, who were in no danger because the explosive materials were "incredibly stable," the Loudoun County sheriff's office said in a statement.

The CIA said it had taken "immediate steps to strengthen inventory and control procedures" in the training program and would conduct a "thorough and independent" review of the incident.

The agency placed the explosive materials inside the engine compartment while training explosives-detecting dogs during spring break, the CIA said.

Some of the material "appears to have been dislodged... and fell into the engine compartment," the sheriff's office said.

A technician conducting routine maintenance on the bus found the explosives on Wednesday.

The CIA has suspended its program until it completes its review, the sheriff's office said.