SANTO DOMINGO/OUANAMINTHE, Haiti: Hundreds of Haitians returned from the Dominican Republic on Thursday after the Dominican president announced an imminent total border shutdown amid a conflict over the construction of a water channel from a shared river.

The border is set to close from Friday at 6 a.m. (1000 GMT) and will last "as long as necessary," Dominican authorities said, with backing from their military and police forces, though talks with the Haitian government are set to continue.

Over a thousand people streamed through the border into Haiti at Ouanaminthe, near the canal, as authorities briefly opened the gates on Thursday afternoon for people getting deported or families looking to return.

Harold Estimable, director of the national migration office in Ouanaminthe, said some 250 to 300 Haitians had been arriving daily from the Dominican Republic in "very bad shape."

United Nations experts warned earlier this week that Haitian women seeking pregnancy and postpartum medical care in the Dominican Republic were reportedly being arrested during check-ups and deported immediately without a chance to appeal.

The Caribbean country has tightened border security amid the worsening gang warfare in Haiti - where there are daily reports of kidnappings and sexual violence - deporting tens of thousands who left their country.

DRASTIC MEASURES

"We have been prepared for weeks, not only for this situation but also for a possible peace force in Haiti," said Dominican President Luis Abinader, adding if the Haitian government could not control the construction, Santo Domingo could.

The Dominican Republic, which threatened to shut the border last week, argues construction works off the River Massacre violate a 1929 treaty.

Abinader is set to raise the issue on a visit to the United Nations next week.

"Unfortunately, they left us no alternative but to take drastic measures," Abinader said, adding the Dominican Republic is planning the construction of two dams that "without the treaty could significantly affect" Haiti.

The government said the border closure will include all land, sea and air routes, and that it deployed a further 20 armored vehicles to a military camp on the border.

Later on Thursday, Haiti's government said that it has the sovereign right to exploit its natural resources, as does the Dominican Republic, in line with the 1929 treaty. It also said it would take all measures to irrigate the Maribahoux plain.

The government said it "will take all necessary measures to protect the interests of the Haitian people."

Local airline Sunrise Airways said it was adding a flight between both countries on Thursday afternoon ahead of the closure.

The U.S. Embassy, which has called on its citizens to leave Haiti, said on its website that those planning to leave for the Dominican Republic would need to make other arrangements.