One day after President Trump appeared to give up on China as an effective mediator in the West’s long-running disputes with North Korea, two of the president’s most senior cabinet officials signaled that the administration has not altogether abandoned the use of China as an intermediary.

“China understands that the United States regards North Korea as our top security threat,” Secretary of State Rex Tillerson told reporters Wednesday. “We reiterated to China that they have a diplomatic responsibility to exert much great economic and diplomatic pressure on the regime if they want to prevent further escalation in the region.”

Tillerson’s comments came after he and Defense Secretary Jim Mattis concluded the first session of a new ministerial-level forum for bilateral exchanges with China that was established by the Trump administration. Senior U.S. officials said they hope the Diplomatic and Security Dialogue, a channel similar to those employed by previous administrations, will be useful for “increasing mutual trust” between the U.S. and Beijing.

The already tense dynamic with North Korea intensified after Otto Warmbier, the 22-year-old Ohio native held captive in North Korea for 17 months on spurious charges of subversion, was returned home last week in a near-comatose state. He died on Monday.

In public remarks, President Trump has appeared visibly moved by the incident, and on Tuesday, he tweeted in frustration: “While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried!”

The Chinese Foreign Ministry responded early Wednesday with a spirited defense of its mediation efforts.

“The crux and focal point of the nuclear issue does not lie with China,” said ministry spokesperson Geng Shuang. “We have been playing an important and constructive role in this regard…and our efforts are indispensable.”

Chinese officials said they had “stepped up” amid recent instability on the Korean peninsula and called on all parties to demonstrate “collective wisdom.”

Tillerson and Mattis met privately with Chinese State Councilor Yang Jiechi and General Fang Fenghui, chief of the People’s Liberation Army’s Joint Staff Department. The Chinese delegation said nothing during a photo opportunity with reporters.

Later, during a brief news conference featuring only the American officials, Mattis spoke bluntly about the Warmbier case – but without him or Tillerson offering any indication as to how they intend to hold North Korea accountable for the student’s death, as numerous U.S. officials have vowed to do.

“We see a young man go over there healthy and -- with a minor act of mischief -- come home dead basically, die shortly -- immediately after he gets here,” Mattis said. “There's no way that we can look at a situation like this with any kind of understanding. This goes beyond any kind of understanding of law and order, of humanity, of responsibility towards any human being.”