Sen. Lindsey Graham (R-S.C.) suggested on Sunday that China may have pressured North Korea to take a harder line against U.S. negotiators during a recent visit to the isolated country by Secretary of State Mike Pompeo.

“I see China’s hands all over this,” Graham told “Fox News Sunday,” citing an ongoing trade war between the U.S. and China as Beijing’s possible motivation. “We’re in a fight with China.”

Graham’s comments came a day after the North Korean foreign ministry released a lengthy statement condemning U.S. negotiators’ “gangster-like” demand for denuclearization.

The criticism suggests that talks between the U.S. and North Korea to end the country’s nuclear program may take longer than President Trump has said. After the president left a landmark summit with North Korean leader Kim Jong Un last month, he declared that Pyongyang was “no longer a nuclear threat.”

Pompeo insisted that his visit to North Korea was “productive.” He fired back at the country’s criticism, saying if the U.S. requests for denuclearization “were gangster-like, the world is a gangster” — a reference to the widespread condemnation of the North’s nuclear ambitions at the United Nations.

Graham placed blame for the North’s colder attitude on Chinese influence, saying that the outbreak of a trade war between Beijing and Washington was to blame for the antagonism.

He also warned China against stepping up the trade conflict, saying that the U.S. has “more bullets than they do when it comes to trade.”

“We can hurt them more than they hurt us,” Graham said, adding that there is “no doubt” in his mind that China is “pulling the North Koreans back.”

“And to our North Korean friends, I can’t say the word ‘friend’ yet,” he added. “You asked Pompeo: ‘Did he sleep well?’ If you knew what I knew about what we could do to the leadership of North Korea, you wouldn’t sleep very well.“