The United States has sent food and medicine to Colombia's border with Venezuela, U.S. officials said on Tuesday, although it is still unclear how the aid will get past the objections of President Nicolas Maduro, who has blocked shipments in the past.Two officials who spoke on condition of anonymity said the aid will be staged at the main Colombian-Venezuelan border crossing at Cucuta. One official said more supplies would be staged in Brazil and in the Caribbean."I anticipate having perhaps a dozen locations all around Venezuela where such aid will be staged," the official said. The U.S. officials said trucks carrying the aid, including high-protein foods, would arrive in Cucuta this week at the request of Venezuelan opposition leader Juan Guaido, who last month declared himself to be the South American nation's interim president. Opposition lawmaker Miguel Pizarro told reporters in Caracas on Tuesday that Guaido's team would talk about how the aid would move once it was in place. Shipments were also coming from Venezuelan companies abroad, Colombia, Canada and Germany.A senior U.S. administration official said it was up to Guaido to decide when and how to move the supplies into the country. "We will seek to help him to do so by whatever means possible," the official said.Prepositioning aid in warehouses or in truck convoys at border posts for weeks, or sometimes months, is common while officials negotiate safe passage.Franklin Graham, chief executive of relief group Samaritan's Purse and the son of renowned Christian evangelist Billy Graham, said until humanitarian groups had access inside Venezuela, most aid likely would remain outside.Samaritan's Purse has operated for the past three years in Cucuta, giving food, medicine and backpacks to Venezuelans heading to Colombia on foot. The group also has managed to get a small amount of aid inside the country, which Graham called "a drop in the bucket" compared to what is needed."The government of Venezuela needs to open the borders, that's it: Open the borders and let food convoys come into your country, open up your airports and let food flights fly in," said Graham, who said he spoke to U.S. President Donald Trump about the situation in December."We are preparing to fly in as soon as we get the green light," he said. Flow of migrantsThe Cucuta crossing was mainly quiet on Tuesday, but police officials told Reuters that more migrants have been asking where humanitarian aid will be delivered.In the nearby border town of La Parada, hundreds of people, some in wheelchairs, lined up in the hot sun to buy medicine. Maria Gomez, a 53-year-old seamstress, arrived from San Cristobal in northern Venezuela to buy her elderly mother the Losartan pills she needs for hypertension."It's cheap here, I always come," she said as a pharmacist handed over the pills. "We cannot buy drugs in Venezuela, you die before you get them."Pressure is growing on Maduro to step down after more than a dozen European Union nations on Monday joined the United States, Canada and a group of Latin American countries in recognizing Guaido as Venezuela's legitimate leader.However Russia, China and Turkey continue to back Maduro, accusing Western nations of meddling.The 35-year-old Guaido, the head of Venezuela's National Assembly, has galvanized the opposition with a hopeful message. He has repeatedly called on Venezuela's military, which has remained loyal to Maduro, to support a transition to democracy.The United States could attempt to seek the approval of the United Nations Security Council to deliver aid without Maduro's cooperation, but Russia would likely block such a move. So far, Maduro has rejected foreign aid. "We are not beggars. You want to humiliate Venezuela, and I will not let our people be humiliated," he said on Monday.Maduro's government, overseeing an economic collapse that has prompted 3 million Venezuelans to flee the country, lashed out at the EU nations for recognizing Guaido, accusing them of submitting to a U.S. "strategy to overthrow the legitimate government."With Maduro in control of Venezuela's military and all the territory, getting aid in will be hard, said Jeremy Konyndyk, a senior fellow at the Center for Global Development, who cautioned against politicizing the assistance."The more it looks like an overt attack on the authority of Maduro, that makes the aid more of a target," said Konyndyk, who has led U.S. government responses to international disasters.