The epicentre of Saturday’s quake was about 12km (7.5 miles) from the town of Saint-Louis du Sud, the US Geological Survey (USGS) said.
The tremor was felt in the densely-populated capital of Port-au-Prince, some 125km away, and in neighbouring countries.
“Lots of homes are destroyed, people are dead and some are at the hospital,” Christella Saint Hilaire, who lives near the epicentre, told AFP news agency.
Prime Minister Ariel Henry said he had mobilised a team to work on the relief effort.
“The most important thing is to recover as many survivors as possible under the rubble,” he said. “We have learned that the local hospitals, in particular that of Les Cayes, are overwhelmed with wounded, fractured people.”
Mr Henry later revealed he had flown in a plane over the city of Cayes.
US President Joe Biden has authorised an “immediate US response” to help Haiti and said USAID would work to support efforts to “assess the damage and assist efforts to recover those who were injured and those who must now rebuild”.
“In what is already a challenging time for the people of Haiti, I am saddened by the devastating earthquake,” he said.
The USGS earlier warned that the earthquake could result in thousands of fatalities and injuries. It also said several aftershocks had been felt in the region, including one measuring 5.8 magnitude.
Frantz Duval, editor-in-chief of Haiti’s Le Nouvelliste newspaper, tweeted that two hotels were among the buildings destroyed in the town of Les Cayes. He said the local hospital was overwhelmed.
“Slowly, strongly and for very long seconds the earth shook in Haiti on 14 August, 2021 around 8:30 am,” he wrote.
Reporters at Le Nouvelliste later said the majority of churches and hotels on the south coast had collapsed or suffered major damage.
Archdeacon Abiade Lozama, head of an Episcopal church in Les Cayes, told the New York Times: “The streets are filled with screaming. People are searching, for loved ones or resources, medical help, water.”
Photos shared on social media showed damaged buildings and piles of rubble after the quake.
Leila Bourahla, Haiti director of Save the Children, told the New York Times it would take days to assess the damage but “it is clear that this is a massive humanitarian emergency”.