Hurricane Matthew has left at least 23 people dead in Haiti, a toll likely to climb as authorities re-establish contact with the hardest-hit areas where the damage is "catastrophic," officials said.

The Caribbean's worst storm in nearly a decade, Matthew slammed into Haiti, the Americas' poorest nation, with heavy rains and devastating winds triggering severe flooding and mud slides.

"The situation in the main cities we flew over was catastrophic," Haiti's interim president Jocelerme Privert said, after surveyed damage to the country's south Wednesday in a US Coast Guard plane.

"Jeremie, Les Cayes, Port-Salut, Petite-Riviere de Nippes, Dame Marie, among others, require all intervention without delay," he said, according to a Wednesday statement.

In addition to the 23 killed, three people remain missing, civil protection spokesman Edgar Celestin said, warning that the toll did not include the department of Grande Anse, which was in the eye of the storm and has been cut off from communication.

Authorities were "briefly able to contact Jeremie (the department capital), but we don't have information from other communities," Celestin said.

More than 21,000 people have been evacuated to temporary shelters.

"We're making every effort to care for these people. The priority is drinking water and food," Celestin said.

Matthew's overall death toll now stands at 27, including four people killed in the Dominican Republic.

-- Cholera cases --

Interior ministry spokesman Guillaume Albert Moleon told AFP that officials "can already see the situation seems very concerning," adding that 25 injuries had been reported.

Flooding brought on by Matthew has sparked a resurgence of cholera, with eight cases already reported.

Torrential rains have left around 2,000 homes flooded and damaged 10 schools, according to the latest partial assessment by authorities.

Both the death and damage tolls are likely to rise as rescue teams reach communities that have been cut off for the last 24 hours.

Initial surveys by helicopter found severely damaged houses, major farmland destruction and intense flooding.

Emergency services have been hampered by the collapse of a bridge, which closed off access to the only road linking Port-au-Prince to the peninsula that makes up southern Haiti.

Overflowing rivers are complicating efforts to bypass access the area.