The quake, which was felt from Istanbul to Athens, hit close to the Turkish resort city of Izmir, which has around three million residents.
“So far, we have received information about six collapsed buildings” in Izmir province, which incudes the city, Turkish Interior Minister Suleyman Soylu said on Twitter.
“Some of our fellow citizens are stuck in the rubble,” said Environment Minister Murat Kurum, adding that he knew of five collapsed buildings.
Images on social media showed water rushing through the streets of Izmir from an apparent sea surge.
The USGS said the quake was registered 14 kilometres (8.6 miles) off the Greek town of Neon Karlovasion on the Aegean Sea island of Samos.
The Turkish government’s disaster agency AFAD reported a lower magnitude of 6.6 for the quake, which struck at a depth of 16.5 kilometres.
Turkish President Recep Tayyip Erdogan tweeted that he was ready to help “with all the means available to our state”.
Turkey is situated in one of the world’s most active earthquake zones.
In 1999, a 7.4 magnitude earthquake struck Turkey’s northwest, killing more than 17,000 people, including 1,000 in Istanbul.
Another quake in 2011 in the southeastern province of Van resulted in more than 600 deaths.