"I had heard that China's giving weapons to, I mean -- you're seeing it all over the place -- to Iran," Trump said in an interview with Maria Bartiromo on Fox Business.

"And I wrote him a letter asking him not to do that, and he wrote me a letter saying that essentially he's not doing that."

Trump's comments come ahead of his planned trip to China on May 14-15 for a summit with Xi that was pushed back from late March by Washington because of the war against Iran.

The conflict in the Middle East has added another aspect of tension in the already complicated US-China relationship. China is a major importer of oil from Iran.

China accused the United States on Tuesday of "dangerous and irresponsible" behavior over its blockade of Iranian ports, with Xi vowing Beijing would play a "constructive role" in promoting peace in the Middle East.

Trump was asked in the interview about reports that China had recently conducted a major cyber attack against the FBI. He did not directly confirm the report, but said: "We do it to them. They do it to us."

"China's China," he said. "They're never easy, but we're doing great with China."

Trump said he was "the toughest person" on China.