WASHINGTON >> The Trump administration hosted senators for an extraordinary White House briefing today 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 today, 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 today 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 today.

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.”

U.S. officials said today’s briefings will center on three key issues: intelligence about the North’s capabilities; U.S. response options, including military ones; and how to get China and other countries to enforce existing economic sanctions on Pyongyang, along with ideas for new penalties. The officials weren’t authorized to speak publicly about plans for the closed-door briefings and requested anonymity.

“China is the key to this,” Sen. John McCain, R-Ariz., said. “The purpose of this briefing is to tell us the situation and the intelligence we have and what (are) the options we have.”

Testifying before the House Armed Services Committee, Harris said he expects North Korea, under Kim’s autocratic rule, to soon be able to develop a long-range missile capable of striking the United States, despite some spectacular failures in its ballistic missile program. “One of these days soon, he will succeed,” Harris said.

North Korea routinely accuses the United States of readying for an invasion, and threatens pre-emptive strikes to stop the U.S.

North Korea’s U.N. mission today said it would react to “a total war” with the U.S. with nuclear war. It said it would win in a “death-defying struggle against the U.S. imperialists.”

A targeted U.S. attack to take out North Korea’s nuclear weapons program could spark a wider war on the Korean peninsula, lawmakers and experts have warned. Harris said the U.S. has “a lot of pre-emptive options,” but he declined to provide specifics in an open setting.

China has been urging restraint by both Pyongyang and Washington. In Berlin, Chinese Foreign Minister Wang Yi said today that North Korea must suspend its nuclear activities, but “on the other side, the large-scale military maneuvers in Korean waters should be halted.”

China opposes the Terminal High-Altitude Area Defense system, or THAAD, being installed in South Korea. The U.S. says it will only target North Korean missiles, but China and Russia see the system’s powerful radars as a security threat.

In Beijing, Chinese foreign ministry spokesman Geng Shuang said THAAD would upset the “strategic balance” in the region. He said China will take “necessary measures to defend our own interests.”

Associated Press writers Hyung-jin Kim and Kim Tong-Hyung in Seoul; Chris Bodeen in Beijing; Richard Lardner, Matthew Lee and Lolita C. Baldor in Washington, and Edith M. Lederer at the United Nations contributed to this report.