At least 21 Venezuelan migrants are missing after a boat they were travelling in capsized on its way to the island of Trinidad and Tobago.
The boat set sail on Tuesday evening but hit choppy waters on its 70km (44-mile) journey to the Caribbean nation.
Four people were rescued in a joint effort by both countries' coast guards.
Since 2014, three million Venezuelans have left their country, which has descended into an economic and political crisis.
The doomed vessel, named Jhonnaly Jose, is reported to have been carrying at least 25 people from Güiria, a port on the north-east coast of Venezuela, although exact passenger numbers remain unclear.
"This tragic incident highlights the extreme risks of sea journeys and other irregular cross-border movements undertaken by refugees and migrants," the UN Refugee Agency, UNHCR, said in a statement.
"It also underscores the desperation of those forced to flee their homes and the extraordinary difficulties faced on their journey," the agency added.
Why are people fleeing Venezuela?
Venezuela's President Nicolás Maduro, who took office in 2013 after the death of Hugo Chavez, has been condemned at home and abroad for alleged human rights abuses and for his handling of the economy.
There are severe shortages of basic items such as medicine and food, and an estimated three million people have fled the country.
The annual inflation rate reached 1,300,000% in the 12 months to November 2018, according to a study by the National Assembly.
President Maduro blames "imperialists" - the likes of the US and Europe - for waging "economic war" against Venezuela and imposing sanctions on many members of his government.