He said he had ordered an immediate investigation into the attack at Phramongkutklao Hospital, said government Spokesman Lt-General Sansern Kaewkamnerd.

The attack hit a dispensary room for commissioned officers on the first floor of the Chalermprakiat building, slightly injuring 24 people. The hospital is situated in central Bangkok on Rajavithee Road near Victory Monument in Ratchathewi district.

Prayut said he had learned about the details of the attack from the defence minister and Army commander in chief.

The attack came on the third anniversary of the coup, which was led by Prayut who was then Army commander in chief.

Sansern said the hospital had already sealed off the scene of the attack pending an investigation.