The Venezuelan military has blockaded a major highway link with Colombia to prevent opponents of the government from bringing food and medicine into Venezuela, a country gripped by life-threatening shortages and a deepening political crisis.

Troops positioned truck trailers and shipping containers to block the lanes of the Tienditas Bridge on the border in northwestern Venezuela on Wednesday.

But it was not clear what effect the blockade, a show of defiance by the government of President Nicolás Maduro, would have. The first convoy of trucks carrying aid from the nearby city of Cúcuta, Colombia, had not yet approached the border, and there were other places it could cross.

The American secretary of state, Mike Pompeo, wrote on Twitter that “the Venezuelan people desperately need humanitarian aid,” and called on the government to let it into the country.