Secretary of State Rex Tillerson and the Trump administration's national security team warned Congress Wednesday of the growing threat posed by North Korea, and said they would be increasing pressure on China to rein in the rogue nation.

"This is an extremely difficult situation, one of the most difficult I have ever seen," Sen. Angus King, I-Maine, a member of the Senate Intelligence and Armed Services Committees, said after senators returned from the meeting. Senators met administration officials in an office building adjoining the White House.

Lawmakers would not provide specific details because the talks were classified, but said the discussion centered on non-military steps that could be taken to reduce the increasing threat that North Korea will use nuclear weapons in the region and eventually, the United States.

"The timeline is getting shorter for them to develop a nuclear weapon that they could deliver here," said Senate Foreign Relations Committee Chairman Bob Corker, R-Tenn. "And you already know they have nuclear weapons."

President Trump attended the briefing but gave only introductory remarks to the lawmakers, they said. The rest of the meeting was conducted by Tillerson and Trump's national security team, who received high praise from senators in both parties.

"One of the strong impressions I came away with is what a good national security team we have," said King, who votes with Democrats.

"I'm convinced [the administration] understand[s] the threat," said Sen. Marco Rubio, R-Fla. "They've established a strategic approach to a very difficult situation."

Sen. Cory Gardner, R-Colo., said the talks centered around the "serious" and "large concern" posed by North Korea and the non-military actions that could deter them from further development of their nuclear weapons program.

"There are so many options that need to be taken that are a long way away from a strike," Gardner said. "There are a number of diplomatic options that can be taken that include secondary sanctions against Chinese individuals, entities and companies that are doing business with North Korea."

Such measures, Gardner said, would pressure the Chinese to "work in good faith" to get North Korea to back off on its aggression.

"The road to a denuclearized North Korea goes directly through Beijing and that is what we have to focus on," Gardner said.

The administration, Gardner said, did not discuss returning the country to the list of state sponsors of terrorism. Gardner, along with Sen. Ted Cruz, R-Texas, have introduced legislation to re-list North Korea.

"It's a serious threat," Rubio said. "Not many easy options."