US to tighten sanctions on North Korea

"The United States seeks stability and the peaceful denuclearization of the Korean peninsula. We remain open to negotiations towards that goal. However, we remain prepared to defend ourselves and our allies," Secretary of State Rex Tillerson, Defense Secretary Jim Mattis and Director of National Intelligence Dan Coats said in a joint statement on Wednesday.

"We are engaging responsible members of the international community to increase pressure on the DPRK in order to convince the regime to de-escalate and return to the path of dialogue," the statement added, using North Korea's official name.

The Trump administration hosted an unusual, hour-long classified briefing on North Korea for all 100 senators earlier in the day - a meeting one Democrat senator described as "sobering."

Tillerson, Mattis and Coats used the meeting to outline North Korea's increasing nuclear capabilities and US response options, officials said.

US preparing military options

Democratic Senator Christopher Coons told reporters that military options were discussed in the briefing.

"It was a sobering briefing in which it was clear just how much thought and planning was going into preparing military options, if called for, and a diplomatic strategy that strikes me as clear-eyed and well proportioned," Coons said.

Ahead of the meeting America's Pacific forces commander, Admiral Harry Harris Jr., told Congress that a controversial missile defense system in South Korea would be operational within days.

He said any North Korean missile fired at US forces would be destroyed, saying "if it flies, it will die."

South Korea began installing key parts of the Terminal High Altitude Area Defense system on Wednesday, sparking Chinese and Russian concerns. Harris warned that North Korean threats needed to be taken seriously and that the US needed to bolster its defenses in Hawaii.

"I don't share your confidence that North Korea is not going to attack either South Korea, or Japan, or the United States ... once they have the capability," Harris told one lawmaker.

North Korea threatens nuclear response

In the past two weeks, US military vessels, including an aircraft carrier, have descended on the region in a show of force.

North Korea's United Nations Mission said if "total war" broke out with the United States it would respond with nuclear options.

"[North Korea will] surely win a victory in the death-defying struggle against the US imperialists," it said in a statement.

The mission said "it is an unshakable will of the DPRK to go to the end if the US wants to remain unchanged in its confrontational stance."

On Tuesday, North Korea conducted large-scale, live-fire artillery drills, witnessed by leader Kim Jong Un.

US Senator Ben Cardin, the Senate Foreign Relations Committee's top-ranking Democrat, said ahead of the briefing that the US needed a strategy to change North Korea's economic and security situation for it to end its nuclear and missile programs.

Meanwhile, China continued its calls for calm on Wednesday. Chinese Foreign Minister Wang Yi said in Berlin on Wednesday that North Korea must suspend its nuclear activities, but "on the other side, the large-scale military maneuvers in Korean waters should be halted."