MEDELLÍN, Colombia — Venezuela’s government said on Wednesday that it was closing its border to air and sea traffic from three Caribbean islands in an effort to block aid shipments to the country organized by the Venezuelan opposition.

The move came ahead of a Saturday deadline by the opposition and the Trump administration for President Nicolás Maduro of Venezuela to end a blockade and allow the delivery of humanitarian aid that has been piling up at the country’s borders for more than a week. The aid showdown is seen as a pivotal moment in the opposition’s effort to force out Mr. Maduro.

Vice President Delcy Rodríguez said that in addition to the border closure, the government would review its relations with the three islands, Aruba, Bonaire and Curaçao, where the Venezuelan opposition wants to stage supplies.

“It’s a sovereign decision,” said Ms. Rodríguez, calling moves to bring the aid in through the islands an attempt by Venezuela’s neighbors to “ignore the legitimate authority of this country.”