WASHINGTON — The Trump administration hosted senators for an extraordinary White House briefing Wednesday at a perilous moment with North Korea, marked by the unpredictable nation’s nuclear threats and stern talk of military action, if necessary, from the United States.

All 100 senators were invited and transported in buses for the unprecedented, classified briefing. President Donald Trump’s secretary of state, defense secretary, top general and national intelligence director were to outline for them the North’s escalating nuclear capabilities and U.S. response options, officials said. The briefing team was to meet later with House members in the Capitol.

The unusual sessions don’t necessarily presage the use of force along one of the world’s most heavily militarized frontiers, and some lawmakers questioned whether the cross-Washington procession was largely show, with Trump expected to drop in on the Eisenhower Executive Office Building gathering of lawmakers.

But it certainly reflected the increased American alarm over North Korea’s progress in developing a nuclear-tipped missile that could strike the U.S. mainland.  And the recent flurry of military activity on and around the divided Korean Peninsula has put the world at high alert.

Tensions have escalated since Trump took office three months ago, determined to halt Pyongyang’s nuclear and missile advances.

In the past two weeks, Trump has ordered high-powered U.S. military vessels, including an aircraft carrier, to the region in a show of force to deter North Korea from more nuclear and missile tests. The North on Tuesday conducted large-scale, live-fire artillery drills, witnessed by national leader Kim Jong Un, as a reminder of its conventional threat to U.S.-allied South Korea.

And on Wednesday, South Korea started installing key parts of a contentious U.S. missile defense system against North Korean missiles that also has sparked Chinese and Russian concerns.

America’s Pacific forces commander, Adm. Harry Harris Jr., told Congress on Wednesday the system would be operational within days. He said any North Korean missile fired at U.S. forces would be destroyed.

“If it flies, it will die,” Harris said.

The Trump administration has said all options, including a military strike, are on the table. However, a U.S. pre-emptive attack isn’t likely, according to American officials. Instead, they’ve said the administration’s strategy focuses on increasing pressure on North Korea with the help of its main trading partner, China.

Sen. Ben Cardin, the Senate Foreign Relations Committee’s top-ranking Democrat, said he was hoping to hear the Trump administration’s game plan Wednesday.

The U.S. needs a strategy to change North Korea’s economic and security calculus for it to freeze and ultimately eliminate its nuclear and missile programs, he said, adding: There’s no “pretty military solution.”