The North Korean situation could spiral out of control, Russia has warned, after another day of inflamed rhetoric from Pyongyang.
Russian Foreign Minister Sergei Lavrov warned of a "vicious circle" and told all sides to avoid unilateral action.
On Thursday, the North threatened to "settle accounts" and said it had put missiles on stand-by to hit the US.
The US, which flew stealth bombers over South Korea this month, condemned the North's "bellicose rhetoric".
White House spokesman Josh Earnest said the rhetoric only deepened North Korea's isolation.
North Korean state media reported leader Kim Jong-un "judged the time has come to settle accounts with the US imperialists".
He was said to have condemned US B-2 bomber sorties over South Korea as a "reckless phase" that represented an "ultimatum that they will ignite a nuclear war at any cost on the Korean Peninsula".
US mainland and bases in Hawaii, Guam and South Korea were all named as potential targets.
North Korea's most advanced missiles are thought to be able to reach Alaska, but not the rest of the US mainland.
'Increasing military activity'
State media in the North showed thousands of soldiers and students at a mass rally in Pyongyang supporting of Kim Jong-un's announcement.
China, North Korea's biggest trading partner, immediately reiterated its call for all sides to ease tensions.
Foreign Ministry spokesman Hong Lei told a daily news conference that "joint efforts" should be made to turn around a "tense situation".
He made similar remarks on Tuesday.
Russian Foreign Minister Lavrov went further, voicing concern that "we may simply let the situation slip out of our control and it will slide into a spiral of a vicious circle".
"We are concerned that... unilateral action is being taken around North Korea that is increasing military activity," he said.
In an earlier statement, the US military said that the B-2 stealth bombers demonstrated America's ability to "provide extended deterrence" to its allies and conduct "long-range, precision strikes quickly and at will".
"The North Koreans have to understand that what they're doing is very dangerous," US Defence Secretary Chuck Hagel told reporters on Thursday.
"We must make clear that these provocations by the North are taken by us very seriously and we'll respond to that."
The US had already flown nuclear-capable B-52 bombers over South Korea earlier this month, in what it called a response to escalating North Korean threats.
Tensions in the Korean peninsula have been high since North Korea's third nuclear test on 12 February, which led to the imposition of fresh sanctions.
North Korea has made multiple threats against both the US and South Korea in recent weeks, including warning of a "pre-emptive nuclear strike" on the US and the scrapping of the Korean War armistice.
While North Korea has issued many threats against the US and South Korea in the past, this level of sustained rhetoric is rare, observers say.
On 16 March, North Korea warned of attacks against South Korea's border islands, and advised residents to leave the islands.
In 2010 it shelled South Korea's Yeonpyeong island, causing four deaths.
On Wednesday, Pyongyang also cut a military hotline with the South - the last direct official link between the two nations.
A Red Cross hotline and another line used to communicate with the UN Command at Panmunjom have already been cut, although an inter-Korean air-traffic hotline still exists.
The jointly run Kaesong industrial park is still in operation.