Opposition leader Juan Guaido, recognized by some 50 countries as Venezuela's interim president, has warned the military that blocking humanitarian aid from entering the country is a "crime against humanity."

The warning comes as international aid has taken center stage in a test of wills between Guaido and President Nicolas Maduro in which the military is seen as the pivotal player.

Medicine and food sent by the United States has been blocked for three days in Cucuta, Colombia after the Venezuelan military closed a bridge linking the two countries.

"There are people responsible for this and the regime should know it," Guaido said after attending Sunday mass with his wife and 20-month-old baby.

"This a crime against humanity, men of the armed forces."

He likewise warned that the military would be held responsible for the deaths of protesters, and reaffirmed his call for a mass march on Tuesday in memory of the estimated 40 people killed in disturbances since January 21.

Maduro has rejected humanitarian aid as a US ploy to intervene in Venezuela, calling the deployment of aid a "political show" and blaming US sanctions for the country's widespread shortages of food and medicine.

Guaido countered that the regime was refusing to acknowledge a "crisis that they themselves generated," while Venezuelans were working to deal with the humanitarian emergency.

Suffering the worst crisis of its modern history, Venezuelans have had to grapple with life-threatening scarcities amid hyperinflation that have rendered salaries and savings worthless.

According to the United Nations, some 2.3 million Venezuelans have fled the country since 2015.

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