Yes, at least according to the North Koreans.
And there are enough reasons for the North Koreans to pronounce this threat as a retaliation to specific recent western actions taken against DPRK, even if the West might not agree on the definition.
- all the data in this answer is based on news reports and articles published until the time of the original posting (2013-04-04 13:08:14Z). If there are updates on the issue that happened after that time, they will not appear here.
The news articles that you cited and many others (CNN The Guardian BI for example) tell the same story "North Korea threatens to attack US main land".
Who is the provoker?
According to the North Koreans they do this as a retaliation to US diplomatic and military steps, source from the Economist:
These are not normal times. The North Korean leadership has reacted bitterly to the latest round of UN sanctions, imposed after its third nuclear test of February 12th. China’s support of the sanctions, which target financial transactions suspected of supporting the North’s nuclear programme, may have increased Pyongyang’s sense of isolation. This week, America singled out North Korea’s Foreign Trade Bank for sanctions, because of its suspected role in nuclear armament.
To add to the tensions, annual joint military exercises between America and South Korea, known as Key Resolve, started on March 11th, involving troop manoeuvres near the demilitarized zone that divides the two countries.
Complicating the situation is the fact of new leadership in South Korea, which threatens to raise the stakes if there is an attack. A spokesman for the defence ministry said on March 12th that any provocation would be met with a response that ensured the North will “suffer far more”. The former president, Lee Myung-bak, faced criticism for failing to order vigorous punishment of the North in November 2010, following a deadly attack on Yeonpyeong, another border island held by the South. The new president, Park Geun-hye, will want to be careful to avoid making a similar impression of weakness.
And there was the 2 B-2 Bombers that flew from the US to South Korea and back, just to show to the North Korean Leadership who has the bigger bomb. source from Reuters
Are they serious?
According to all the western media sources, North Korea doesn't have missiles that can reach the US main land. From the NYT source:
Most analysts do not believe that North Korea has a missile powerful enough to deliver a nuclear warhead to the United States mainland or that it is reckless enough to strike the American military in the Pacific.
And there are even pictures:
source: The Guardian
On the other hand, North Korea have published a picture with an attack plan on US main land:
The text apparently reads "U.S. Mainland Strike Plan" (which is not subtle). The four cities that are targeted are Austin, San Diego, Honolulu, and Washington DC.
source: Business Insider
And the USA takes at least some precautions:
the United States announced Wednesday that it was speeding the deployment of an advanced missile defense system to Guam in the next few weeks, two years ahead of schedule in what the Pentagon said was “a precautionary move” to protect American naval and air forces from the threat of a North Korean missile attack.
from the NYT article
Lastly, there is an article by Foreign Affairs, which I found very interesting, that say that if a conflict breaks out, it'll quickly escalate to a nuclear conflict by the North Korean.