Responding to the North Korea "war mongering" on strategic ally South Korea, the US said it took the threats with due seriousness.
"We´ve seen reports of a new and unconstructive statement from North Korea. We take these threats seriously and remain in close contact with our South Korean allies," said Caitlin Hayden, spokeswoman for the White House National Security Council.
"But, we would also note that North Korea has a long history of bellicose rhetoric and threats and today´s announcement follows that familiar pattern," she added.
The US, however, said that it has taken complete preparations to defend themselves.
"We remain fully prepared and capable of defending and protecting the United States and our allies," said Hayden.
"We continue to take additional measures against the North Korean threat, including our plan to increase the US ground-based interceptors and early warning and tracking radar," said she.
Earlier on Saturday, North Korea said it has entered a ´state of war´ with South Korea.
"From this moment, the north-south relations will be put at the state of war and all the issues arousing between the north and the south will be dealt with according to the wartime regulations," said a North Korean government statement as carried by state-run Korean Central News Agency (KCNA).
KCNA said the statement was issued by the North Korean government, political parties and other organizations.
"The state of neither peace nor war has ended on the Korean Peninsula," read the statement.
"Now that the revolutionary armed forces of the DPRK have entered into an actual military action, the inter-Korean relations have naturally entered the state of war. Accordingly, the DPRK will immediately punish any slightest provocation hurting its dignity and sovereignty with resolute and merciless physical actions without any prior notice," it said.
"If the U.S. and the South Korean puppet group perpetrate a military provocation for igniting a war against the DPRK in any area including the five islands in the West Sea of Korea or in the area along the Military Demarcation Line, it will not be limited to a local war, but develop into an all-out war, a nuclear war," the statement said.
Reacting to America´s use of nuclear-capable B-2 bombers in joint South Korean military drills, North Korea on Friday threatened to attack the United States military bases in South Korea and Pacific and put its rocket units on standby.
During a midnight meeting with top generals, North Korea´s supreme leader Kim Jong Un signed the plan on technical preparations of strategic rockets of the KPA (Korean People´s Army), ordering them to be standby for fire so that they may strike any time the U.S. mainland, its military bases in the operational theaters in the Pacific, including Hawaii and Guam, and those in south Korea.
KCNA said: "He (Kim Jong Un) said he has judged the time has come to settle accounts with the U.S. imperialists in view of the prevailing situation."