Donald Trump has threatened to impose an additional 5% tariff on Mexico if it does not immediately provide additional water to help US farmers, accusing the country of violating a treaty that outlines water-sharing between the neighbors.

Under the treaty, which dates back to 1944, Mexico must send 1.75m acre-feet of water to the US from the Rio Grande through a network of interconnected dams and reservoirs every five years. The agreement also stipulates that the United States must annually allocate 1.5m acre-feet of water from the Colorado River to Mexico.

The water from the Rio Grande is used to support agriculture in Texas and other US border states.