President Donald Trump said late on Monday he did not believe that China and Russia were helping Iran "at all," adding that Moscow and Beijing had "too much to lose" if they enabled Tehran.
Trump said on Truth Social that he had seen comments from his critics and opponents that China and Russia were helping Iran.
"If they are, it's at a very low level, but I don't believe they are at all. They have too much to lose, and we would know about it, and know about it very fast," Trump wrote.
Trump said in July he trusted China and Russia not to help Tehran in the Iran war, but warned that if they did get involved "it would be very bad for them."
The United States and Iran have been at war since February 28 when the US and Israel attacked Iran and Tehran responded with its own strikes on Israel and Gulf states that host US bases. US-Israeli strikes on Iran and Israeli attacks on Lebanon have since killed thousands and displaced millions.
Trump has previously described Chinese President Xi Jinping and Russian President Vladimir Putin as "neutral" during the Iran war.
Moscow and Beijing maintain close ties with Tehran. Russia has said the Iran war could lead to a nuclear arms race in the Middle East. Beijing has condemned Washington's attacks on Tehran as a brazen violation of Iran's sovereignty.
Last month, Trump warned Xi against providing support to Iran when the Chinese leader visited Washington, and received assurances that China would not do so, the US ambassador to China said at the time.
The warning came amid reports that China had supplied Iran with shoulder-fired air-defense missile launchers and that Tehran had found ways to evade sanctions on its oil sales with Chinese help.
In September, Trump brushed aside a report that Iran acquired satellite images of a base in Jordan from Chinese entities before an attack that killed three US troops there in July.