U.S. President Donald Trump on Tuesday ordered a “blockade” barring all sanctioned oil tankers from entering or leaving Venezuela, in the latest move by Washington to increase pressure on President Nicolas Maduro’s government by targeting its main source of revenue.

It remains unclear how the Trump administration will enforce the blockade or whether it will deploy the U.S. Coast Guard to intercept vessels, as it did last week.

The decision comes after the United States deployed thousands of troops and about 12 warships, including an aircraft carrier, to the Caribbean region in recent months.