WASHINGTON (AFP) - Three US Marines who were briefly hospitalised after a mysterious envelope was opened at a base near Washington were well enough to report for duty on Wednesday (Feb 28), a spokeswoman said.

Agents with the FBI and the Naval Criminal Investigative Service (NCIS) were still examining the envelope and its contents, which arrived late Tuesday at Joint Base Myer-Henderson Hall.

Shortly after receiving the letter, 11 people started to feel ill, the Marine Corps said.

Three of them were "transported in stable condition for further medical evaluations" to a local hospital.

Base spokeswoman Leah Rubalcaba told AFP the three Marines were back on duty early Wednesday and the affected headquarters building had been "cleared for repopulation."

She said the NCIS and FBI had not yet disclosed what might have been in the envelope.

CNN reported that a corporal, a gunnery sergeant and a colonel complained of a "burning sensation" on their hands and face.

Marine spokesman Major Brian Block said it was too soon to say whether the incident was being considered a terror attack.

"It's way too early to start speculating on any of that, we have to wait and let the investigators do their jobs," Block told AFP.

Joint Base Myer-Henderson Hall overlooks Arlington Cemetery just across the Potomac River from Washington.

Its status as a joint base means it has Navy, Marine Corps and Army commands.