Secretary of State Rex Tillerson has warned China that U.S. military action against North Korea is a possibility unless China works with the Trump administration to rein in the regime's nuclear weapons program, he revealed Wednesday.

He conveyed that warning during a recent dialogue that brought Tillerson and Defense Secretary Jim Mattis into a room with Chinese State Councillor Yang Jiechi and representatives of the Chinese military. Tillerson was blunt about the stakes of the North Korea crisis, which has been exacerbated in recent decades by Chinese economic support.

“I said, ‘State councillor, if you and I don't solve this, these guys get to fight,'” Tillerson recalled to former Secretary of State Condoleezza Rice during a discussion at the Hoover Institution in Stanford, Calif. “And we don't want that, and neither do you.”

Such discussions inform Tillerson’s assessment that a rigorous international sanctions campaign could force North Korea to begin major nuclear talks. But he also defended the aggressive military posture that China and Russia criticized as an escalation of the threat in the region.

“When we get to that negotiating table — and I'm confident we will — I want to know that Secretary Mattis has a very, very strong military option standing behind me,” Tillerson said. “That will give me a better position from which to try to solve this.”

Some U.S. lawmakers are skeptical that China will allow North Korea to face the kind of crippling economic pressure that would force dictator Kim Jong Un to reconsider his push to develop nuclear weapons and ballistic missiles capable of striking the United States.

“We have to be willing to risk our trade relationship with China to get a level of cooperation from China on this issue that goes beyond the foreign policy decision that they have made,” Rep. Brad Sherman, D-Calif., said during a House Foreign Affairs subcommittee hearing Wednesday afternoon.

Chinese diplomats, for instance, rebuffed Tillerson’s request for an oil embargo on North Korea. But China did agree to restrict North Korea’s ability to buy oil. Tillerson noted to Rice that North Korean fishing boats are being sent out in winter, to try to ameliorate food shortages in the country. Approximately 100 boats have drifted into Japanese territorial waters.

“They're being sent out to fish with inadequate fuel to get back,” he said Wednesday. “And so, we're getting a lot of evidence that these sanctions are really starting to hurt.”