At least 227 people were killed and hundreds were injured and missing after a 7.2 magnitude earthquake struck Haiti on Saturday.
Prime minister Ariel Henry said he was sending aid to areas where towns were destroyed and hospitals overwhelmed with incoming patients.
The epicentre of the quake was about 78 miles west of the capital of Port-au-Prince, the US Geological Survey said, and widespread damage was reported.
Haiti's civil protection agency said that the death toll stood at 227 and that search teams would be sent to the area.
Rescue workers and bystanders were able to pull many people to safety from the rubble, the agency said Saturday on Twitter.
It said injured people were still being taken to hospitals.
Mr Henry declared a one-month state of emergency for the whole country and said he would not ask for international help until the extent of the damages was known.
He said some towns were almost completely razed and the government had people in the coastal town of Les Cayes to help plan and co-ordinate the response.
"The most important thing is to recover as many survivors as possible under the rubble," said Mr Henry.
"We have learned that the local hospitals, in particular that of Les Cayes, are overwhelmed with wounded, fractured people."
He said the International Red Cross and hospitals in unaffected areas were helping to care for the injured, and appealed to Haitians for unity.
"The needs are enormous. We must take care of the injured and fractured, but also provide food, aid, temporary shelter and psychological support," he said.
Later, as he boarded a plane bound for Les Cayes, Mr Henry said he wanted "structured solidarity" to ensure the response was co-ordinated to avoid the confusion that followed the devastating 2010 earthquake, when aid was slow to reach residents after as many as 300,000 were killed.
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