The quake, which occurred roughly 125 km (78 miles) west of Port-au-Prince on Saturday, triggered landslides and the total collapse of two communities. Several aftershocks have also been felt.
Around 7,000 homes were destroyed and nearly 5,000 damaged, including hospitals, schools, offices and churches in what is the poorest country in the western hemisphere.
Footage taken from above on Sunday showed structures turned to rubble, and swathes of towns such as Les Caynes, on the Haitian coast, almost turned to dust.
Rescuers were also seen searching for survivors before the arrival of a tropical storm, Grace, which was expected to reach Haiti on Monday night.
Officials said it could bring strong winds, heavy rain, rough seas, landslides and flooding - which will hamper rescue efforts even further.
Henrietta Fore, the executive director for Unicef, told reporters that many Haitians were in desperate need of medical attention, clean water and shelter - with families in Les Caynes forced to sleep on a football field after the collapse of their homes.
In Martissant, on Haiti's southern coast, local officials were forced to negotiate with local gangs to allow a UN convoy of aid and humanitarian workers though.
Prime minister and acting president Ariel Henry told reporters on Sunday he was declaring a state of emergency for the whole of Haiti following the quake, which was the worst in 11 years.
"We salute the dignity, the resilience effort of the victims and their ability to start over," said Mr Henry, as the first aid convoys reached the worst-affected communities.
"From my observations, I deduce that Haitians want to live and progress. Let us unite to offer these people a living environment conducive to development."
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