Speaking on Fox News' Sunday Morning Futures with Maria Bartiromo, Trump said: "If we catch them doing that, they get a 50 percent tariff, which is a staggering, that's a staggering amount."

The warning adds a new layer of tension ahead of Trump's planned visit to Beijing next month for talks with President Xi Jinping, a summit already postponed once because of the US-Iran conflict.

Tariff threats expanded to China after talks fail

The remarks followed Trump's earlier declaration that any nation supplying Iran with military equipment would face immediate economic penalties. "A Country supplying Military Weapons to Iran will be immediately tariffed, on any and all goods sold to the United States of America, 50%, effective immediately. There will be no exclusions or exemptions," he wrote on Truth Social.

While Trump's initial warning did not explicitly name China, he clarified over the weekend that Beijing was included, days after the US-Iran ceasefire negotiations in Islamabad collapsed without a deal.

In a series of Truth Social posts, he said the talks had hit a wall because Iranian officials were "unyielding" on their nuclear programme, although some progress had been made on other fronts.

Beijing has rejected accusations that it is supplying weapons to Tehran, according to a Reuters report.

Washington hardens stance as concerns over Beijing grow

US officials have grown increasingly wary of China's strategic posture in the region. Analysts told Reuters that the conflict with Iran is already straining Washington's foreign-policy bandwidth, potentially complicating Trump's preparations for the rescheduled meeting with Xi.

Trump sharpened his rhetoric, telling reporters, "If China does that, China is gonna have big problems," citing intelligence warnings that Beijing may be preparing shipments of weapons to Iran.

A CNN report, quoting sources familiar with recent assessments, said China is believed to be planning deliveries of advanced air-defense systems in the coming weeks, potentially routed through third countries to obscure their origin. The systems reportedly include shoulder-fired anti-air missiles, or MANPADs, capable of threatening low-flying military aircraft.

Both China and Russia have faced longstanding accusations of bolstering Iran's defences, though both deny involvement in any recent transfers.

Tariffs re-emerge as Trump's leverage despite legal limits

Trump's tariff threats come despite a February ruling by the US Supreme Court, which struck down his broadest use of the International Emergency Economic Powers Act, limiting the president's ability to impose sweeping global tariffs under the 1977 law.

Still, hours after agreeing to a two-week ceasefire with Tehran earlier in the week, Trump returned to tariffs as a pressure tool. "A Country supplying Military Weapons to Iran will be immediately tariffed…" he posted, reaffirming the warning first issued mid-week.

The conflict, now in its sixth week, has included extensive US and Israeli strikes against Iran's missile sites, military infrastructure and weapons industry. Intelligence concerns about possible resupply efforts by China or Russia surged as the bombardment intensified.

Beijing and Moscow reject allegations

China and Russia have both denied providing weapons to Iran during the current conflict. However, US officials have continued to monitor what they describe as suspicious activity involving potential arms transfer channels.

Both nations have historically supported Iran's military development, including through the supply of air-defence platforms and missile systems, but appear to have limited such assistance during the latest US-Israeli operations.

(With inputs from agencies)