The US is sending a missile shield to the Pacific island of Guam as North Korea threatens nuclear strikes.
The Pentagon said the shield would be ready within weeks, adding to warships that were sent to the area earlier.
The North had named Guam among a list of possible targets for attack that included Hawaii and the US mainland.
North Korea is not thought to have the technology to strike the US mainland with either a nuclear weapon or a ballistic missile, analysts say.
But it is capable of targeting US military bases in the region with its mid-range missiles.
Pyongyang has also continued to refuse access to workers from the South into a joint industrial zone in the North.
The Kaesong complex is staffed mainly by North Koreans but funded and managed by South Korean firms.
Pyongyang blocked access for a second day on Thursday, and threatened to shut down the zone.
North Korea has issued an array of statements in recent weeks threatening nuclear strikes and attacks on specific targets in the US and South Korea.
It has announced a formal declaration of war on the South, and pledged to reopen a mothballed nuclear reactor in defiance of UN Security Council resolutions.
In its latest statement, attributed to a military spokesman, the North appeared to refer to ongoing military exercises between the US and South Korea in which the US has flown nuclear-capable bombers over the South.
The statement said the "ever-escalating US hostile policy towards the DPRK [North Korea] and its reckless nuclear threat will be smashed".
It promised to use "cutting-edge smaller, lighter and diversified nuclear strike means of the DPRK" and said the "merciless operation of its revolutionary armed forces in this regard has been finally examined and ratified".
The US Department of Defense said on Wednesday it would deploy the ballistic Terminal High Altitude Area Defense System (Thaad) to Guam in the coming weeks.
The Thaad system includes a truck-mounted launcher and interceptor missiles.
The Pentagon said the deployment would "strengthen our regional defence posture against the North Korean regional ballistic missile threat".
The US floated plans to send a Thaad system to Guam in 2009, but never followed through.
US officials recently also announced that the USS John McCain, a destroyer capable of intercepting missiles, had been positioned off the Korean peninsula.
Analysts have expressed concern that it is unclear exactly what Pyongyang hopes to achieve with its latest round of ramped-up rhetoric.
The BBC's Damian Grammaticas in Seoul says the North could be seeking to pressure Washington to open fresh talks.
US Defense Secretary Chuck Hagel said Washington was taking the threats seriously.
"As they have ratcheted up her bellicose, dangerous rhetoric, and some of the actions they've taken over the last few weeks present a real and clear danger," said Mr Hagel, in his first major speech since taking up his post.
The North has not taken direct military action against its neighbours since 2010, when it shelled a South Korean island, killing four people.