DAJABON, Dominican Republic (AP) — The Dominican Republic shut all land, air and sea borders with Haiti on Friday in a dispute about construction of a canal on Haitian soil that taps into a shared river, as armed Dominican soldiers patrolled entry points and military planes roared overhead.

Flights were canceled and border towns usually teeming with vendors and Haitians crossing daily to work in the Dominican Republic were subdued. Crowds of people on the Haitian side gathered under the shade of trees as they observed the scene on Friday. Nearby, a white flag fluttered in the breeze under a Haitian flag in a sign of peace.

It was unclear how long the rare closure of the borders will last, with Dominican President Luis Abinader saying the measure will remain in place “as long as necessary.” The country’s Ministry of Foreign Affairs said in a statement that the canal project violates a 1929 treaty and “must be halted immediately before pursuing any other dialogue.”

Abinader ordered his administration to buy all perishable goods normally exported to Haiti, including chicken, onions, beans and eggplants. The food will be used for government programs that offer free meals to students and others, according to Joel Santos, minister of the presidency.

“Producers should know that the government is going to support them in this situation, because the measure taken by the president represents an issue of security and defense of national sovereignty,” he said.

The diplomatic crisis began earlier this month when workers in Haiti resumed construction of a canal near the Massacre River that runs along the border, to help alleviate a drought that hit Haiti’s Maribaroux plain. The river is named after a bloody clash between Spanish and French colonizers in the 18th century, and was the site of a mass killing of Haitians by the Dominican army in 1937.

Abinader says the canal will divert water and negatively affect Dominican farmers and the surrounding environment, while Haiti’s government insists that building the canal falls within its sovereign right to decide how to use its natural resources.

The closure will represent a significant economic hit for both countries that share the island of Hispaniola, although Haiti is expected to feel it more acutely.

“It’s really a very drastic measure that doesn’t make sense economically for either the Dominican Republic or Haiti,” said Diego Da Rin with the International Crisis Group. “This will clearly have very bad consequences economically in the Dominican Republic, and it will very likely worsen the humanitarian situation mostly in the areas close to the border.”

Haiti is the Dominican Republic’s third biggest trading partner, with $1 billion in exports to Haiti last year and $11 million in imports, according to the Dominican Republic’s Export and Investment Center.

Meanwhile, a study by the Dominican Republic’s Central Bank found that $430 million in informal border trade was conducted in 2017 between the countries. Of that amount, more than $330 million consisted of exports to Haiti.

Officials from the two countries met on Wednesday to discuss the situation, and were still meeting on Thursday when Abinader announced he would close all borders on Friday, prompting the Haitian government to criticize what it called a “unilateral” decision.

Da Rin called Abinader’s actions an overreaction and noted that he confirmed last month he is running for re-election, and appeared to be staking out tough stance on migration. “Maybe Abinader thinks this is a way to portray himself as a strong nationalist leader who will be the only one ... able to really stop the ‘Haitian invasion’ as he always calls the growing migration influx.”

Authorities were still allowing people to cross into Haiti on Friday, although the long lines from Thursday had diminished.

Among those debating whether to leave the Dominican Republic was Jhon Alberto, who sells purses and men’s clothing in the northern city of Santiago de los Caballeros.

“My parents are asking me to leave. They don’t want me to have a problem here,” he said, adding that he was concerned for his safety if he stays.

Many Haitians, like Julián Jean, 50, who works at a chicken farm in the Dominican Republic that exports animals to Haiti, questioned why the borders were closed.

“Over a little canal so we can gather a bit of water?” he said. “(It’s) so we can progress and see if we find a bit of food for the stomach because we are going through a lot of hunger and a lot of work.”

On Friday, Haiti’s Support Group for Returnees and Refugees condemned Abinader’s moves, and said the canal work should continue.

“Closing the border will bring big consequences for Haitian migrants,” coordinator Ketia Bronté said.

She warned that more people are going to cross the border illegally and that the number of cases of human trafficking and contraband would likely increase.

“Haiti and the Dominican Republic are two nations whose history is intertwined,” she said. “Their destiny is linked to living together on an island.”

Abinader announced this week that he has stopped issuing visas to Haitians and closed the border near the town of Dajabon. He also has pushed to limit the number of Haitians migrating to the Dominican Republic and has expelled tens of thousands of them and those of Haitian descent. Bronté noted that in August alone, some 22,000 Haitians were deported — twice the usual monthly number.

The Dominican Republic also has started building a 118-mile (190-kilometer) wall along the Haitian border that he announced early last year.

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Coto reported from San Juan, Puerto Rico. Associated Press videographer Pierre-Richard Luxama in Port-au-Prince, Haiti, contributed.