WASHINGTON -- North Korea conducted live-fire artillery drills and a U.S. guided-missile submarine arrived in South Korea on Tuesday, escalating the standoff over the North's nuclear weapons program as the Trump administration prepared an extraordinary White House briefing for senators.

Fears North Korea could mark the 85th anniversary of its military's founding with a nuclear test explosion or a ballistic missile launch proved unfounded. But the unpredictable communist nation rattled its saber all the same, with drills that served as a reminder of the threat it poses below the border to U.S.-allied South Korea.

The exercise in the area of east coast city of Wonsan involved 300 to 400 artillery pieces, South Korea's Yonhap news agency said. An official from Seoul's Defence Ministry couldn't confirm such details. Seoul lies only 25 miles from the demilitarized zone separating the two Koreas, well within artillery range.

President Donald Trump has sent more U.S. military assets to the region in a show of force while leaning on China to exert economic pressure on its wayward ally. Chinese President Xi Jinping, who spoke to Trump on Monday, is urging restraint from both Pyongyang and Washington.

In Washington, top Trump administration officials are due to brief the entire U.S. Senate on Wednesday. A rapid tempo of North Korean weapons testing in the past year has pushed Kim Jong Un's authoritarian nation closer to developing a nuclear-tipped missile that could reach the U.S. mainland.

Republican Sen. Lindsey Graham voiced confidence Tuesday that Trump won't allow North Korea to reach that point. Graham, a defence hawk who dined with Trump on Monday night, said the North should not underestimate the president's resolve.

"We are probably in one of the most challenging situations since the Cuban missile crisis," Sen. John McCain, another Republican who joined Trump for the dinner, told a congressional hearing Tuesday, referring to the 1962 standoff with the Soviet Union that pushed the superpowers close to nuclear confrontation. McCain said a North Korean nuclear missile capable of striking an American city was "an imminent danger."

McCain said Trump is "exploring all options" on North Korea. A pre-emptive strike, he said, "would be the last one."

The USS Carl Vinson aircraft carrier is headed toward the Korean Peninsula and will hold a joint exercise with South Korea. However, the deterrence effect of the operation may have been undermined by confusion over when the carrier arrives. The deployment was announced more than two weeks ago.

In the meantime, the USS Michigan, a nuclear-powered submarine, arrived Tuesday at the South Korean port of Busan for what was described as a routine visit to rest crew and load supplies. The U.S. 7th Fleet said two American destroyers were conducting simultaneous maritime exercises with naval ships from South Korea and Japan.

At the Senate Armed Services Committee hearing Tuesday, U.S. lawmakers probed experts on the potential consequences of a pre-emptive U.S. military strike on North Korea. They heard sobering responses.

Princeton University professor Aaron Friedberg said North Korea could begin with a massive artillery barrage against Seoul, and unleash special forces and chemical and biological weapons, even if that would lead to the annihilation of Kim Jong Un's North Korean dictatorship.

"A conflict on the peninsula would be unlike anything we have seen in decades," Kelly Magsamen, a former senior U.S. defence official, said. "North Korea is not a Syria, it's not an Iraq."

"The consequences could be extremely high," she said, warning that China could intervene.

Graham surmised there are "no good choices left."

But he said, "if there's a war today, it's over there. In the future if there's a war and they get a missile it comes here." Left unsaid by Graham was that a war today could be disastrous for U.S. allies Japan and South Korea.

North Korea routinely accuses the United States of readying for an invasion. On Monday, North Korean Defence Minister Gen. Pak Yong Sik threatened to use pre-emptive strikes or any measures it deems necessary to defend itself against "U.S. imperialists."

The streets of Pyongyang, however, were quiet for Tuesday's anniversary, which was overshadowed by April 15 celebrations for the birthday of the nation's late founder Kim Il Sung and were marked by a missile test the following day.

The Trump administration is also upping the ante diplomatically.

On Friday, Secretary of State Rex Tillerson will chair a special meeting of the U.N. Security Council.

Tillerson will be "very vocal" about nations enforcing sanctions on North Korea, State Department spokesman Mark Toner said. Trump said Monday the council must be prepared to impose stronger sanctions.

Kim Tong-Hyung in Seoul, South Korea, Eric Talmadge in Pyongyang, North Korea, and Richard Lardner in Washington contributed to this report.