Chuck Hagel, the US defense secretary, has said North Korea's provocative actions and belligerent tone has "ratcheted up the danger" on the Korean peninsula.
"We have to take seriously every provocative, bellicose word and action that this new young leader has taken so far," Hagel told a Pentagon news conference on Thursday, referring to Kim Jong-un.
"We will be prepared - we have to be prepared - to deal with any eventuality," he said.
But Hagel denied that the United States had aggravated the situation by flying stealth bombers to the region.
Washington deployed two nuclear-capable stealth bombers over South Korea as part of a "deterrence" mission over the Korean peninsula.
The deployment of B-2 bombers on Thursday followed North Korea's decision to cut a military hotline with the South, breaking the last direct communication link between the two countries at a time of heightened military tensions.
The bombers made the 20,000km round trip from the US state of Missouri to South Korea in a single flight.
Al Jazeera's Harry Fawcett, reporting from Seoul, said the US was sending "a very clear message to North Korea".
Fawcett also reported that US defense secretary Charles Hagel has called his South Korea counterpart "to reaffirm the commitment" of the US to defend its ally.
On Wednesday, the North said it would inform the UN Security Council that the threat of nuclear war in the region was growing.
"Under the situation where a war may break out any moment, there is no need to keep up North-South military communications," the official was quoted as saying by the official Korean Central News Agency. "From now, the North-South military communications will be cut off".
In response to the latest provocation, the US said it was ready to respond to "any contingency".
"North Korea's bellicose rhetoric and the threats that they engage in follow a pattern designed to raise tensions and intimidate others," Jay Carney, White House spokesman, said.
Several weeks ago, North Korea severed the Red Cross hotline that had been used for government-to-government communications in the absence of diplomatic relations.
Despite the cutting of the military hotline, the Seoul-funded Kaesong industrial complex in North Korea continued to operate as of Thursday.
The industrial estate, established in 2004 as a symbol of cross-border co-operation, has remained operational despite repeated crises in relations.
The North has also threatened war against the South and its ally the US, saying conditions "for a simmering nuclear war" have been created on the peninsula.
"This is because of provocation moves by the US and South Korean puppets," the statement said.
In another sign of brewing tensions, a South Korean soldier standing on guard at the inter-Korean border threw a grenade towards a moving object in the dark early on Wednesday, causing a short-lived alarm.
At daylight, a patrol searched the area but there was no trace of any infiltration from North Korea, a South Korean ministry spokesman said.
A precautionary alert, which had been issued for South Korean units in the northeastern county of Hwacheon, was consequently lifted.
Earlier in the day, the North had repeated threats to target US military bases.
Pyongyang said its military would put all field artillery units, including long-range artillery units and strategic rocket units, into combat duty position that would target all "enemy objects" in the US, "invasionary" bases on its mainland, Hawaii and Guam.
The rhetoric from North Korea drew more concern from China, Pyongyang's only major ally, which said the situation was "sensitive".
In an interview with Al Jazeera, Leonid Petrov, a Korea expert at Sydney's Australian National University, said North's "attention-seeking behaviour" was in response to it feeling "cornered" by the international community.
"The regime wants the people of North Korea to be consolidated behind its young leader Kim Jung-un," Petrov said.
On the other hand, Petrov said, the North was also hinting that it was ready to negotiate.
"Pyongyang really wants to have a peaceful, diplomatic solution to the growing problem," he said.
South Korea and the US military are conducting military drills until the end of April, which they have stressed are strictly defencive in nature.