Venezuelan military officers blocked a bridge on the border with Colombia ahead of an anticipated humanitarian aid shipment, as opposition leader Juan Guaido stepped up his challenge to President Nicolas Maduro's authority.

Earlier on Tuesday the opposition-dominated National Assembly had warned the armed forces, which make up much of Maduro's power base, not to cross a "red line" by blocking aid.

Guaido, who proclaimed himself acting president on January 23 - sparking an international crisis - claims that up to 300,000 people face death if the aid is not delivered.

"You know there's a red line, you know well there's a limit, you know that medicines, food and medical supplies are that limit," lawmaker Miguel Pizarro said in a message to the military.