Five Iranian tankers apparently consisting of at least $45.5 million in value of gasoline and similar items are currently sailing to Venezuela in defiance of U.S. sanctions, reported The Associated Press on Sunday.

A senior U.S. official told Reuters on May 14 that the Trump administration is reviewing measures it could take in response to the shipment.

“It is not only unwelcome by the United States but it’s unwelcome by the region, and we’re looking at measures that can be taken,” the official told the outlet.

Amid economic collapse, Venezuela has been relying on Iran to prop up its oil industry by giving the Islamic Republic tons of gold bars, thereby depleting the South American country’s gold vaults, reported Bloomberg late last month, citing sources with direct knowledge of the matter.

What was once an oil-rich nation, Venezuela holds some 70 tons of gold in its vaults, according to Bloomberg.

In a letter to U.N. Secretary-General António Guterres, Iranian Foreign Minister Mohammad Javad Zarif said that the United States must “give up bullying on the world stage,” accusing the Trump administartion of “piracy” in threatening the “disruption of Iran’s fuel transmission to Venezuela.”

Iranian Deputy Foreign Minister Abbas Araqchi echoed Zarif and told Swiss Ambassador to Iran Markus Leitner that any U.S. action against the tankers would be met with an “immediate and firm response.”