Treasury Secretary Scott Bessent said the administration had cautioned governments and banks that continued transactions involving Iranian oil could trigger punitive measures. “If you are buying Iranian oil, that if Iranian money is sitting in your banks, we are now willing to apply secondary sanctions,” he told reporters at the White House.

The warning comes as the U.S. enforces a naval blockade on Iran that began earlier this week, with officials saying they expect the move to curb purchases by key buyers, particularly China, which has accounted for the majority of Iran’s seaborne oil exports.

Bessent said the Treasury had contacted two Chinese banks, warning that they could face sanctions if evidence shows Iranian funds moving through their systems. China’s embassy in Washington did not immediately respond to requests for comment.

Separately, the U.S. Treasury announced sanctions targeting Iran’s oil transportation network, including more than two dozen individuals, companies and vessels, as part of a broader effort to disrupt the country’s energy exports.

The measures follow the administration’s decision not to renew a 30-day sanctions waiver issued on March 20 that had allowed Iranian oil shipments already at sea to reach global markets.

The waiver, which Bessent said facilitated the delivery of about 140 million barrels, is set to expire on April 19.Washington has also allowed a similar waiver covering Russian oil shipments to lapse, reinforcing its broader sanctions enforcement strategy.

U.S. officials said letters had been sent to jurisdictions including China, Hong Kong, the United Arab Emirates and Oman, identifying financial institutions allegedly linked to Iranian transactions and warning of potential enforcement actions.

The steps are part of what the administration has described as a “maximum pressure” campaign aimed at curbing Iran’s oil revenues and limiting its ability to finance regional activities.

The two allies agreed that Seoul does not fall under the manipulator designation that the U.S. Treasury Department announces in reports twice a year, Kang Yu-jung told a press conference

Officials at the U.S. embassy in Seoul could not be reached for comment outside business hours.

The administration of President Joe Biden added South Korea to a manipulation monitoring list in November due to its large current account surplus and its sizable trade surplus with the U.S. The government of Donald Trumpkept Seoul on the list in June.

Under a 2015 U.S. law,Washington can take “remedial action” against countries that do not “correct the undervaluation of their currency and trade surplus with the United States”.

The South Korea-U.S. deal is not related to talks on a currency swap as part of bilateral negotiations over Trump’s tariffs on South Korean goods, South Korean officials said.

President Lee told Treasury Secretary Scott Bessent on Wednesday in New York that the Asian country needs a foreign exchange swap in order to make the $350 billion investment it has pledged in the tariff talks, Finance Minister Koo Yun-cheol said on Saturday.

Koo quoted Bessent as saying he would discuss the issue with other U.S. officials and get back to South Korea.

South Korean National Security Adviser Wi Sung-lac reiterated on Saturday that Seoul cannot pay the $350 billion “upfront”, as Trump has suggested in recent days. President Lee told Reuters this month that South Korea’s economy could fall into crisis rivalling its 1997 meltdown if the government accepted the U.S. demands without safeguards.

Koo said he had not heard anything about a Wall Street Journal report that Commerce Secretary Howard Lutnick had discussed raising the $350 billion investment.