WASHINGTON — President Trump and his top national security advisers briefed senators Wednesday at the White House on what a senior aide called the “very grave threat” posed by North Korea, as the administration continued to develop a range of economic and military measures to pressure Pyongyang.

“Nothing is risk-free. This situation is not risk-free,” the senior administration official told reporters in the White House briefing room as the meeting with the senators was underway at a secured location next door at the Eisenhower Executive Office Building. “But the team has done everything we can try to anticipate reactions and mitigate the risk.”

Trump offered to host the meeting – open to all 100 senators, who arrived on a large white bus – as a courtesy after Senate Majority Leader Mitch McConnell, R-Ky., asked for the briefing in response to increased tension on the Korean Peninsula, officials said. The U.S. military had redirected the USS Carl Vinson aircraft carrier strike group toward the region in response to a failed North Korean missile test last week.

Senators expressed frustration as they left the briefing that the administration shared few details of its current policy on North Korea and its plans to deal with the country as it continues developing its nuclear weapons program.

“They’re trying to do the right thing,” said one Republican senator, but members of both parties left frustrated that they were given “very few details about what has changed.”

The briefing lacked “even straight answers on what the policy is regarding North Korea and its testing of ICBMs,” said the senator, who requested anonymity to speak frankly about the meeting.

“Several senators asked specifically, ‘What is the policy?’ and the briefers gave us very, very few details,” the senator said.

Trump and Vice President Pence briefly addressed the senators at the beginning of the meeting.

When they left, senators heard from Secretary of State Rex Tillerson, Defense Secretary Jim Mattis, Daniel Coats, the national intelligence director, and Gen. Joseph Dunford Jr., chairman of the Joint Chiefs of Staff. Just a handful of staffers were in the room with the four briefers.

In a joint statement, Tillerson, Mattis and Coates called North Korea’s pursuit of nuclear weapons “an urgent national security threat and top foreign policy priority.” They said Trump’s approach aimed tao tighten economic sanctions and pursue “diplomatic measures” with allies and partners.

The goal is to “convince the regime to de-escalate and return to a path of dialogue” toward peaceful denuclearization of the Korean Peninsula, they said. “We remain open to negotiations to towards that goal. However we remain prepared to defend ourselves and our allies.”

The Republican senator said “the basic gist of it at the beginning was that we’re going to get more aggressive, we’ve waited and they’ve continued to be bad actors. We’ve reached a point where things are getting pretty dire and getting to the point where we’ve got to get more aggressive.”

“From then on, what we all wanted to know is, What does that mean?” the senator added. “What is it that we should be looking for as the trigger that something is about to happen and that we’d end up taking some kind of kinetic action? That’s where things got a little elliptical.”

Several senators said that while the briefing was sobering, it was not revelatory.

“There was very little, if anything new,” said Sen. Richard Blumenthal, D-Conn. “I remain mystified about why the entire Senate had to be taken over to the White House rather than conducting it here.”

The briefing for the senators on White House grounds was a departure from normal procedure, when such briefings are held in secure, underground auditoriums or conference rooms at the U.S. Capitol that are built to withstand digital eavesdropping.

Senators were instructed to leave all their electronics outside, “which at least left the suggestion with everyone that this was a classified environment,” the senator said.

“There was a definite degree of resolve that we’ve got a bad situation on our hands and they’re ratcheting up the importance of this,” the Republican senator said. “One of the things that I surmised from it was that as much as anything else, perhaps they wanted to prepare everybody for the fact that this could escalate quickly. That’s my own read on it.”

Sen. Dan Sullivan, R-Alaska, went in planning to speak to Trump’s team about his intention to pull colleagues together to draft a bill implementing a “much more robust missile defense” against the potential threat of North Korean strike against the United States – as well as a policy that would allow overwhelming retaliation in the case of such a strike. “I certainly don’t think that they think it’s a bad idea,” Sullivan said afterward.

But he too described the meeting as an effort to get senators versed on the administration’s position, in the hopes they would then be effective spokespeople for that policy.

“Particularly the Senate gets out, travels the world, meets with world leaders,” Sullivan said.

Still, lawmakers that went in hoping to push the administration to take intermediary steps – such as stiffening sanctions against China for its support of Pyongyang, or re-labelling North Korea as a state sponsor of terror – came away with no promises the administration would soon take any such steps.

Cory Gardner, R-Colo., stressed the administration already has the tools it needs to come down harder on North Korea, such as imposing harsher sanctions that Congress gave them the authority to impose last year, and re-listing North Korea as a state sponsor of terror.

“North Korea’s far from being sanctioned out,” Gardner said. “We should take the steps necessary including secondary sanctions on Chinese entities to bring pressure on them so that they can start working in a good faith manner toward denuclearization on the peninsula.”

“I have supported putting North Korea back as a state sponsor of terror,” he added. “That’s something I support – but no indication yet from the administration.”