Turkey’s rocky relationship with the United States has hit fresh turbulence over Ankara’s gold exports to Venezuela. Washington has aired worries that the flourishing trade may be in violation of US sanctions against President Nicolas Maduro’s embattled regime and is threatening to take action against its NATO ally. The Donald Trump administration has instructed US Assistant Treasury Secretary for Terrorist Financing Marshall Billingslea to broach the subject with Turkish authorities during a previously planned trip to Ankara to discuss US sanctions on another Washington bugbear, Iran.

Billingslea has previously voiced concerns about Venezuela’s gold imports to Turkey. “We are looking at the nature of Turkish-Venezuelan commercial activity,” a senior US official told Reuters, “and if we assess a violation of our sanctions, we will obviously take action.” Venezuela exported more than 20 metric tons of gold to Turkey last year and there are suspicions that some of it may have wound up in Iran’s hands.

Why it matters: Last year, Venezuela exported 23.6 tons of gold worth $900 million to Turkey, compared to zero the previous year. Critics say Turkey’s facilitation of Venezuelan gold exports, which have thrown a lifeline to Maduro’s collapsing regime, is not unlike the help Ankara extended to Tehran in an oil-for-gold scheme that the FBI described as the biggest sanctions-busting scam the agency had ever uncovered. That investigation led to the sensational March 2016 arrest of Turkish-Iranian gold trader Reza Zarrab, who became a key witness for the prosecution in the federal government’s case against Mehmet Hakan Atilla, a senior executive for Turkish state lender Halkbank. The bank was allegedly at the nexus of the money laundering operation.

Turkey has been pushing back against a potential multibillion-dollar fine being mulled by the Treasury Department. Ankara allegedly linked the release of detained US pastor Andrew Brunson to ongoing negotiations with Washington on the issue. Brunson was released in October after the Trump administration slapped sanctions on Turkey’s interior and justice ministers and raised tariffs on Turkish aluminum and steel imports, sending the lira into a tailspin.