Five people, including three tourists, were reported missing early Friday after an explosion tore through a building in Athens' historic Plaka district, just below the Acropolis.
The blast on a small street shook the area, which is normally busy with hundreds of foreign visitors.
The fire service said they were looking for two elderly people who lived on the building's top floor, and three tourists believed to have rented the apartment below.
"A search and rescue operation with a dog is underway to find anyone who may be trapped," a fire department spokeswoman told AFP.
"We have learnt that an elderly couple and three other people staying in a holiday rental were living in the building and we are trying to determine whether they were there at the time of the explosion," she added.
Excavators cleared debris and thermal cameras were being used by rescuers, footage from the public television channel ERT showed.
ERT said investigators believe the explosion, which occurred at around 7:30 a.m. (0430 GMT) and preceded the collapse of the building, was caused by a gas leak.
According to information provided to the rescue teams and reported by the newspaper Kathimerini, the second floor of the building was occupied by an elderly couple and the first floor by three American tourists, in a short-term rental.
Earlier, two women were rescued from an adjacent building and taken to hospital. Another person was slightly injured by debris.
Neighbouring buildings and vehicles parked nearby were damaged.
The owner of the ground floor apartment, Mathaios Artavanis, interviewed by Kathimerini, said he had tried to contact "the elderly couple, around 70 years old, since the morning" without success.
Athens city planning services were mobilised to check the stability of nearby buildings, Mayor Haris Doukas told a local radio station.
The historic Plaka district, where the building is located, is one of the most touristy areas of Athens.
It is near Hadrian's Arch, one of the main monuments from the Roman period in the Greek capital, where the tourist season continues to be in full swing.