The explosion occurred on Friday morning in an older two-story building in the tourist area of Plaka at the foot of the Acropolis rock in the Greek capital.
Authorities on Friday evening increased the number of missing people from five to six. They include an elderly couple who lived on the top floor of the house and tourists from the United States who are believed to have rented the apartment below.
Two women were taken to hospital after being rescued from a nearby building, and another person was slightly injured by debris. Several nearby buildings and cars were also damaged.
Attempts to contact the six people who are feared to be trapped in the rubble have failed.
