The Turkish government is coming under renewed pressure to stop buying gold from Venezuela, the BBC has learned.
A senior Western diplomatic source told the BBC that Turkey was now considered the main concern among the countries engaging in the trade.
They added that there are growing suspicions that gold exported to Turkey was ending up in Iran, which would violate US sanctions.
Turkish President Recep Tayyip Erdogan backs Venezuelan President Nicolás Maduro, who is facing a growing political challenge.
Venezuela's gold trade with Turkey is soaring. Last year, the country exported almost $900m (£688m) worth of gold to Turkey, ostensibly to be refined there and returned to Venezuela, although there is no record of re-exportation.
American government officials have previously warned that some of the gold may be heading via Turkey to Iran, which would violate US sanctions.
The BBC has been told that Ankara has received fresh warnings about potential sanctions-busting.
Moreover, Turkey is now seen by Western governments as posing by far the biggest concern regarding the trade, over and above Russia and the United Arab Emirates, both of which are also reportedly receiving the gold.
On Friday, US Senator Marco Rubio, a member of the Foreign Relations Committee, warned the UAE and Turkey not be "accomplices" in the "outrageous crime" of shipping gold out of Venezuela.
Russian and Turkish private jets appear to have travelled to Caracas in recent days. Ankara insists its trade is in accordance with international regulations.
One of the private jets spotted travelling to Caracas in recent days belonged to Ciner group, a major Turkish mining company with close ties to President Erdogan.
A Venezuelan opposition MP, José Guerra, claims a Russian plane belonging to the company Nordwind landed in Caracas with the aim of transporting at least 20 tonnes of gold out of the country.
The Venezuelan government is accused by opponents of extracting gold in an illegal and environmentally damaging way, contaminating pits and running organised crime networks to control small miners, often with severe violence.
President Donald Trump has signed an executive order banning US nationals from engaging in gold trade with Venezuela. That could be extended to third parties such as Turkey; the authorities in Ankara have been warned that they are skating on thin ice, sources told the BBC.
Political relations between Turkey and Venezuela have flourished since 2016, with four visits to Turkey by Mr Maduro and a reciprocal one to Venezuela by President Erdogan in 2018 - the first ever by a Turkish head of state.
It comes as Venezuela's oil revenue is starved by sanctions and a drop in the oil price, forcing Caracas to seek revenue through other means, namely gold.
Venezuela's Industry Minister Tareck El Aissami visited a Turkish gold refinery in Corum, near Ankara, last month.