The effort involves plans to send weapons through other countries in an effort to hide the origins of the shipments.
Chinese companies have been discussing arms sales with Iran, plotting to send the weapons through other countries to mask the origins of the military aid, according to U.S. officials.
The United States has gathered intelligence that Chinese companies and Iranian officials have discussed the arms transfers. It is not clear how many, if any, arms have been shipped or to what degree Chinese officials have approved the sales.
The new disclosure is likely to intensify pressure on President Trump to raise the issue while he is in Beijing this week. But whether Mr. Trump will remains a question. While he has pressured leaders of smaller countries during their visits to the White House, Mr. Trump seems intent on resetting the relationship with President Xi Jinping during his meetings in China.
Mr. Trump said on Tuesday that he planned to have a “long talk” with Mr. Xi about the conflict in the Middle East and added that the Chinese leader had been “relatively good” on Iran.
Officials briefed on the intelligence have reached different conclusions on whether the arms have already been sent to the third countries. But no Chinese weapons appear to have been used on the battlefield against U.S. or Israeli forces since they began their war against Iran in late February.
The New York Times reported last month that U.S. intelligence agencies had obtained information showing that China may have transferred shoulder-fired missiles, known as MANPADS, to Iran. Such weapons are capable of shooting down low-flying aircraft. Intelligence also showed that China was considering other shipments of the weaponry.