Trump issued a dire warning to Venezuela's military that if they continue to stand with Maduro, "you will find no safe harbour, no easy exit and no way out. You will lose everything."

Trump added: "We seek a peaceful transition of power, but all options are open."

Trump's move followed a whirlwind of high-profile visits to the Venezuelan-Colombia border by US officials — including Senator Marco Rubio, and the head of the US Agency for International Development — who have embraced the opposition's plans to use aid as their chief political weapon.

If Maduro's stranglehold on the food and medicine supply can be broken, and he can be shown to have lost control of the border, his legitimacy as the country's president will weaken, the reasoning goes. If the military can be convinced to not stand between the Venezuelan population and the humanitarian aid, he may fall.

"Food and medicine will reach the people of Venezuela and so will liberty and democracy," said Rubio, standing in front of the Tienditas Bridge in Cucuta, which Maduro's government blocked with a couple of shipping containers.

It is a risky gamble for Maduro's adversaries, who say they are unclear as to how they will break the blockade at the border on Saturday. Maduro still controls the military, and with it, the state.

"If the opposition — and Trump administration — are trying to find ways to peel away military support for Maduro, threatening its monopoly on food distribution is not likely to be helpful in that regard," said Cynthia Arnson, the Latin America director at the Woodrow Wilson International Centre for Scholars.

She added that by creating a political showdown over the humanitarian shipment, the White House only increased the prospects that Maduro would keep blocking the aid.

For Guaido, there is an additional risk: in accepting wholeheartedly Trump's embrace, Guaido risks looking like a puppet of the United States.

"Being associated too closely with the US carries too much baggage anywhere in Latin America," said Adam Isacson of the Washington Office on Latin America, a human rights group. "And when you have a leftist government that rallies its base on these issues of American imperialism, it plays closely into their narrative."

Isacson, who directs the group's security and defence program, also expressed concern about the tension that the rhetoric of Trump and Maduro was raising on the border.

"With this kind of saber-rattling you create potential for a hair-trigger moment," he said, citing Maduro's dispatch of troops to the border. "An incident there could go badly."

Cucuta, which is across a river from Venezuela, now offers the kind of contradictory scenes that reflect a country where two men claim the presidency. One of the shipping containers blocking the bridge into Venezuela is emblazoned with the word "paz" Spanish for "peace".

On the Venezuelan side, the government has amassed soldiers, militiamen, armoured vehicles and even missiles. On the Colombian side sit news camera crews and trucks full of supplies. Richard Branson, the British billionaire, has invited a lineup of Latin American musicians to perform an aid concert on Friday night.

Yet no one knows how the aid will reach Venezuela. The opposition has so far been quiet about details of its plans, saying that if they released information Maduro would stymie them with his security forces.

But Gaby Arellano, an opposition lawmaker sent by Guaido to coordinate the aid, said that the opposition did not necessarily have to use the blocked Tienditas Bridge.

"The border with Colombia is immensely long, and so is the border with Brazil, and the border with the Antilles," she said. "We want the aid to be coming in at all points."

For more than a week, activists and officials have said they are mulling the option of simply smuggling in aid through Venezuela's porous land borders, along routes long used to transport contraband products and fuel. Opposition activists have said they have already joined forces with the Pemones indigenous community in eastern Venezuela to bring in supplies by river, using their canoes.

Another option, pushed by those looking for a more direct confrontation with Maduro, would have activists circle an aid truck in Colombia as it slowly makes its approach to Venezuela.

Under this plan, protesters from Venezuela would overrun soldiers stationed on the Venezuelan side and allow the aid to move in, possibly using a forklift to push aside the containers blocking the bridge.

In the Carribean nation of  Curacao, opposition officials were buoyed by the willingness of the country's foreign minister to stage aid along a sea corridor long used by Venezuelan migrants to flee the country. But in recent days, plans appeared to be falling apart as Curacao politicians objected to the use of the aid as a political weapon.

The uncertainty has left some in Venezuela not counting on aid anytime soon.

"They say they are in charge of the government, but the ones who are in charge are the ones who control the bridge," said Hector Cardenas, 52, who crossed the border to buy a month's worth of cooking oil, vegetables, soap and medicines in Colombia. "The opposition has no real power."

The New York Times, AP