US President Donald Trump on Thursday threatened sanctions on anyone who buys Iranian oil, a warning that came after planned talks over Tehran’s rapidly advancing nuclear program were postponed.

Trump wrote on social media, “All purchases of Iranian Oil, or Petrochemical products, must stop, NOW!” He said any country or person who buys those products from Iran will not be able to do business with the United States “in any way, shape, or form.”

It was unclear how Trump would implement such a ban, as he threatened to levy secondary sanctions on nations that import Iranian oil. But his statement risked further escalating tensions with China -- Iran’s leading customer -- at a time when the relationship is severely strained over the US president’s tariffs.

Based on tanker tracking data, the US Energy Information Administration concluded in a report published last October that “China took nearly 90 percent of Iran’s crude oil and condensate exports in 2023.” Trump has separately placed 145 percent tariffs on China as a way to raise federal revenues and rebalance global trade.

Trump's social media threat came after Oman announced that planned nuclear negotiations for this coming weekend had been postponed. Omani Foreign Minister Badr al-Busaidi made the announcement in a post on the social platform X.

“For logistical reasons, we are rescheduling the US-Iran meeting provisionally planned for Saturday, May 3rd,” he wrote. “New dates will be announced when mutually agreed.”

Al-Busaidi, who has mediated the talks through three rounds so far, did not elaborate.

Iran's Foreign Ministry spokesperson Esmail Baghaei issued a statement describing the talks as being “postponed at the request of Oman’s foreign minister.” He said Iran remains committed to reaching “a fair and lasting agreement."

Meanwhile, a person familiar with the US negotiators said America “had never confirmed its participation” in a fourth round of talks in Rome. However, the person said the U.S. expected the talks to occur “in the near future.” The person spoke on condition of anonymity to discuss the closed-door negotiations.

Rome will see the Vatican begin its conclave next week to pick a new pope after the death of Pope Francis. Two other rounds of US-Iran talks have been held in Muscat, the capital of Oman.

The talks seek to limit Iran’s nuclear program in exchange for the lifting of some of the crushing economic sanctions the US has imposed on the Islamic Republic, closing in on a half-century of enmity. Iranian Foreign Minister Abbas Araghchi and US Mideast envoy Steve Witkoff have led the negotiations.

Trump has repeatedly threatened to unleash airstrikes targeting Iran’s program if a deal isn’t reached. Iranian officials increasingly warn that they could pursue a nuclear weapon with their stockpile of uranium enriched to near weapons-grade levels.

The US president, with his threat on social media, would be going after Iran's major economic resource. It produced an average of 2.9 million barrels a day of crude oil in 2023, according to the Energy Information Administration.

Iran’s 2015 nuclear deal with world powers did limit Tehran’s program. However, Trump unilaterally withdrew from it in 2018, setting in motion years of attacks and tensions. The wider Middle East also remains on edge over the Israel-Hamas war in the Gaza Strip.

Meanwhile, the US continues an airstrike campaign, called “Operation Rough Rider,” that has been targeting Yemen's Houthi rebels, who long have been backed by Iran. US Defense Secretary Pete Hegseth warned Iran over the rebels early Thursday.

“Message to IRAN: We see your LETHAL support to the Houthis. We know exactly what you are doing,” he wrote. “You know very well what the US Military is capable of -- and you were warned. You will pay the CONSEQUENCE at the time and place of our choosing.”

Last Saturday's round of talks, which included experts drilling down into the details of a possible deal, also took place as an explosion rocked an Iranian port, killing at least 70 people and injuring more than 1,000 others.