A small tsunami struck the Seferihisar district south of Izmir, the city in western Turkey that was the worst affected by the quake, said Haluk Ozener, director of the Istanbul-based Kandilli Observatory and Earthquake Research Institute.
At least 12 people were killed in Izmir, Turkey's third-largest city, including one who drowned, according to Turkey's Disaster and Emergency Management Presidency, or AFAD.
Environment and Urbanization Minister Murat Kurum said 522 people were injured, with five undergoing surgery and eight in intensive care.
On Samos, two teenagers died after being struck by a wall that collapsed. Greek Prime Minister Kyriakos Mitsotakis tweeted condolences, saying "Words are too poor to describe what one feels before the loss of children."
At least 19 people were injured in Samos, with two, including a 14-year-old, being airlifted to Athens and seven hospitalized on the island, health authorities said.
The small tsunami that hit the Turkish coast also affected Samos, with seawater flooding streets in the main harbor town of Vathi. Authorities warned people to stay away from the coast and from potentially damaged buildings.
Izmir Gov. Yavuz Selim Kosger said at least 70 people were rescued from wrecked buildings, with four destroyed and more than 10 collapsed. Others suffered less severe damage, he said, but did not give an exact number.
Search-and-rescue efforts were underway in at least 17 buildings, AFAD said. Turkish media showed rescuers pulling people from the rubble, including one survivor who was found about six hours after the quake. Emergency teams continued digging after nightfall.
The earthquake, which the Kandilli Institute said had a magnitude of 6.9, struck at 2:51 p.m. local time (1151 GMT) in Turkey and was centered in the Aegean northeast of Samos. AFAD said it measured the magnitude at 6.6.
It was felt across the eastern Greek islands and as far as the Greek capital, Athens, and in Bulgaria. In Turkey, it shook the regions of Aegean and Marmara, including Istanbul. Istanbul's governor said there were no reports of damage in the city, Turkey's largest.
Video on Twitter showed flooding in the Seferihisar district, and Turkish officials and broadcasters called on people to stay off the streets after reports of traffic congestion. Izmir Mayor Tunc Soyer urged residents to not enter damaged buildings and to be mindful of social distancing and mask mandates amid the COVID-19 pandemic.
Clouds of dust or smoke rose from several spots as buildings collapsed in the quake.
Greek seismologist Akis Tselentis told Greek state broadcaster ERT that due to the shallow depth of its epicenter - roughly 10 kilometers - potentially powerful aftershocks could be expected for several weeks and warned that buildings could collapse in a strong aftershock.
The government and cities like Istanbul sent more than 2,000 rescue personnel to Izmir, as well as relief supplies. The Turkish Red Crescent set up kitchens.
France offered assistance to both countries. The secretary of state for European affairs tweeted France's "full solidarity with Greece and Turkey" and said "we are ready to offer the necessary aid."
The Greek minister responsible for civil protection and crisis management, Nikos Hardalias, headed to Samos along with a search-and-rescue team, paramedics and engineers.
In a show of solidarity rare in recent months of tense bilateral relations, Greek and Turkish government officials issued mutual messages of solidarity.
"We pray that there is no further loss of live in Turkey or Greece and we send our best wishes to all those affected on both sides of the earthquake," Turkey's Communications Director Fahrettin Altun tweeted. "This tragedy reminds us once again how close we are despite our differences over policy. We're ready to help if Greece needs it."
Mitsotakis, the Greek prime minister, tweeted that he had phoned Turkey's President Recep Tayyip Erdogan "to offer my condolences for the tragic loss of life from the earthquake that struck both our countries. Whatever our differences, these are times when our people need to stand together."
Erdogan responded to the tweet with his thanks and offered his condolences. "Turkey, too, is always ready to help Greece heal its wounds. That two neighbors show solidarity in difficult times is more valuable than many things in life," he wrote.
Relations between Turkey and Greece have been particularly tense, with warships from both facing off in the eastern Mediterranean in a dispute over maritime boundaries and energy exploration rights. The ongoing tension has led to fears of open conflict between the two neighbors and NATO allies.