Tighter US sanctions are killing off Turkey’s gold-for-gas trade with Iran and have stopped state-owned lender Halkbank from processing other nations’ energy payments to the OPEC oil producer, bankers said on Friday.

India is among the countries that has used Halkbank to pay for Iranian oil imports.

US officials have sought to prevent Turkish gold exports, which indirectly pay Iran for its natural gas, from providing a financial lifeline to Tehran, largely frozen out of the global banking system by Western sanctions over its nuclear programme.

Turkey, Iran’s biggest natural gas customer, has been paying Iran for its imports with Turkish lira, because sanctions prevent it from paying in dollars or euros.

Iranians then use those lira, held in Halkbank accounts, to buy gold in Turkey, and couriers carry bullion worth millions of dollars in hand luggage to Dubai, where it can be sold for foreign currency or shipped to Iran.

Halkbank had also been processing a portion of India’s payments for Iranian oil.

A provision of US sanctions, implemented from February 6, tightens control on sales of precious metals to Iran and prevents Halkbank from processing oil payments by other countries back to Tehran.