The United States has warned Nicolas Maduro that his “days are numbered”, ahead of an international meeting in Bogota on Monday with his rival, Juan Guaido, to decide the next steps in the battle for Venezuela.

Mike Pompeo, the secretary of state, who will travel to Bogota alongside Vice-President Mike Pence, condemned the weekend's violence and said he was confident Mr Maduro would soon be gone.

"Predictions are difficult. Picking exact days is difficult," he said, speaking on a US political chat show the day after a deadly standoff over aid shipments erupted at the borders of the South American nation.

"I'm confident that the Venezuelan people will ensure that Maduro's days are numbered."

The violence looked set to increase, however. Venezuela's prisons minister, Iris Varela, on Saturday afternoon announced that she was bringing "trained" inmates from the country's overcrowded and highly dangerous prisons to the border, to "have the privilege of fighting for the fatherland".

"I'm bringing them here, and I will continue to bring them here, in significant numbers," she said, speaking to reporters in the Venezuelan border city of San Antonio.