To prevent further spread of the coronavirus, the land border restrictions between the United States, Canada, and Mexico have been extended through at least September 21.

In a tweet, Department of Homeland Security Acting Secretary Chad Wolf said, “We continue to work with our Canadian and Mexican partners to slow the spread of #COVID19. Accordingly, we have agreed to extend the limitation of non-essential travel at our shared land ports of entry through September 21.”

Restrictions on nonessential travel between the North American countries were set to expire on August 21, but that deadline has been extended by another month. The countries closed their borders to nonessential travel in March, with extensions occurring every month since.

Reuters reported that the restrictions don’t apply to trade or travel by air.

A version of this article was previously published on June 17, 2020, and was updated on August 17, 2020, with more information.