US President Donald Trump has ordered a ban on all oil tankers entering and leaving Venezuela.

The president tweeted that the government of Venezuelan President Nicolas Maduro has been designated a foreign terrorist organization and accused of stealing American assets and engaging in "terrorism, drug trafficking, and human trafficking."

"Therefore, effective today, I have ordered a complete blockade of oil tankers entering and leaving Venezuela," he said.

The president's order comes a week after the United States seized an oil tanker off the coast of Venezuela.

The Venezuelan government issued a statement rejecting Trump's "vague threats."

In a social media post, Trump said Venezuela was "completely surrounded by the largest naval force ever seen in the history of South America."

He added that the military siege was "going to get worse" and threatened to be "unlike anything you've seen before." Trump accused Maduro's government of using stolen oil to "fund itself."

He added that "drug trafficking is being used for terrorism, human trafficking, murder, and kidnapping."

The Trump administration has repeatedly accused Venezuela of drug trafficking, and since September, the US military has killed at least 90 people in raids on boats it suspects were transporting fentanyl and other illegal drugs to the United States.