The U.S. has announced that firms shipping via Mexico must now prove the origins of their products if they want to avoid the border taxes.

BBC News reports that the new rules, which were jointly announced with Mexican President Andres Manuel Lopez-Obrador, target attempts to evade tariffs by routing shipment of steel and aluminum through Mexico to try to evade tariffs.

The measures expand protections for U.S. steel and aluminum makers that were launched under former president Trump in 2018, which set a 25% tax on steel and 10% tax on aluminum imports.

Mexico won an exemption, but now the White House has decided to revisit that, as an increase in imports from the country suggested it was being used as a stopping point to bypass the taxes.

Companies shipping steel to the U.S. must now prove it has been melted and poured in North America. For aluminum, firms must prove it has not been smelted in China, Russia, Belarus or Iran.

Only about 13% of the steel imported from Mexico last year — and only 6% of aluminum — originated from outside of North America, according to officials.