CUCUTA, Colombia (Reuters) - A U.S. military transport plane carrying humanitarian aid meant for Venezuelans landed on Saturday in the Colombian border city of Cucuta, where food and medicine is being stored amid uncertainty over how and where aid will be distributed.

The shipment will be the second arrival of large-scale U.S. and international aid for Venezuelans, many of whom have scant access to food and medicine, since opposition leader Juan Guaido declared himself interim president in defiance of socialist President Nicolas Maduro.

Guaido, who argued Maduro’s 2018 re-election was a sham and invoked constitutional provisions to declare himself the country’s leader last month, has said aid will enter Venezuela on Feb. 23.

But it remains unclear whether Maduro, who has called the aid a U.S.-orchestrated show and denies any crisis, will allow the supplies to cross into Venezuela.

Speaking in Caracas to supporters who had volunteered to help with the aid effort, Guaido said he would announce details on Monday about how he planned to get aid into the country from Colombia, Brazil and Curacao despite Maduro’s opposition.

“We will organize ourselves into brigades,” Guaido said, calling on the military to allow the aid through. “The message we have to get through to the armed forces is that they have one week to do the right thing. Will you be on the side of your family and your people or of the usurper who keeps lying?”

Millions of Venezuelans will be traveling to the border to safeguard arriving aid, Lester Toledo, a Guaido representative, said at a news conference in Cucuta.

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“We are seven days from this being a reality,” Toledo said. “We are going to have the accompaniment of people, of hundreds of thousands, of millions of Venezuelans that our president Juan Guaido has called upon, who we have asked to go to the border dressed in white as a sign of peace.”

A U.S. official told Reuters the aid delivered on Saturday amounted to more than 200 tonnes but a U.S. Embassy representative said they had no measurements of the shipment. Toledo said three planes would eventually arrive.

Most Western countries and many of Venezuela’s neighbors have recognized Guaido as the legitimate head of state, while Maduro retains the backing of Russia and China and control of Venezuelan state institutions including the military.

Acting Defense Secretary Patrick Shanahan said on Saturday that the United States used military aircraft to send aid to the Venezuelan border in Colombia because of the urgency of the humanitarian needs.

“It’s a message to Venezuela that we are supporting their humanitarian needs,” Shanahan told reporters, adding the aid was being transported by three C-17 aircraft.

The supplies, including hygiene kits and special products meant for children suffering malnutrition, are arriving from a U.S. Air Force base in Florida, the U.S. Embassy said in a statement.

Additional aid flights will take place over the coming days, the statement said, and medical supplies and pharmaceuticals meant for use in hospitals will arrive early next week.

The first aid shipment, which included basic foodstuffs and medical supplies, arrived Feb. 8 and is being stored in a Cucuta warehouse.

Two bridges outside Cucuta that mark the border between the two countries were open as normal on Saturday. A bridge outside Tienditas, Venezuela, which has never been used, remains blocked on the Venezuelan side by shipping containers.