Saying diplomacy was difficult, President Donald Trump warned Thursday that the United States "could end up having a major, major conflict with North Korea. Absolutely."

"We'd love to solve things diplomatically but it's very difficult," Trump told Reuters in an Oval Office interview in which he identified North Korea as his biggest global challenge.

His comments came a day after summoning U.S. lawmakers to the White House for a rare—and seemingly pointless—briefing on North Korea, and one day ahead of Secretary of State Rex Tillerson calling on the United Nations Security Council to implement sanctions, suspend, or downgrade diplomatic relations with the hermit kingdom in an effort to "denuclearize" the nation.

Failing to do so, he said Friday, would be "catastrophic," as he warned that the U.S. was willing to use military force.

"All options for responding to future provocations must remain on the table," Tillerson said. "Diplomatic and financial leverage or power will be backed up by willingness to counteract North Korean aggression with military action, if necessary."

Also Thursday, North Korea released a new video simulating attacks on the White House and U.S. aircraft carriers.

Trump's latest instance of saber-rattling contrasted with remarks made earlier Thursday by Tillerson, who told NPR in an interview that the U.S. would prefer to resolve the North Korean conflict through direct talks. Tillerson's tone also differs from that of Vice President Mike Pence, who talked tough on North Korea earlier this month—reflecting internal disconnect that may, as CNN noted, "leave allies uncertain."

Or, as U.S. Rep. Ted Lieu wrote on Twitter: "The inconsistent messages being sent by [the Trump] administration on North Korea show confusion, not strength."

For its part, China warned Friday that the situation with North Korea is "under very grave tension and at a critical point," with diplomacy as the only way to diffuse such tensions.

"A peaceful settlement of the nuclear issue on the Korean Peninsula through dialogue and negotiations represents the only right choice that is practical and viable," Chinese Foreign Minister Wang Yi told reporters on Friday.

Meanwhile, diplomacy also appears to be the approach favored by the majority of South Koreans. According to a poll by the Korea Society Opinion Institute released Wednesday, nearly seven in 10 respondents want the next government (the country is holding an election May 9) to resume exchanges with Pyongyang in the interest of peaceful inter-Korean relations.

Earlier this week, a letter from 40 women leaders to Trump demanded he take steps to avert nuclear war—including by initiating a peace process with North Korea, South Korea, and China.