"The real question, though, is the degree to which China will be willing to implement the U.N. sanctions and to impose punishment of its own.
"A sharp drop in Chinese grain sales to North Korea in January may be a sign that China's support for U.N. sanctions is more than just a symbolic punishment."
Fitzpatrick characterized North Korea's reported threat to nullify the 1953 armistice as "largely bluster," pointing out that the country has "broken the armistice many times, most recently in 2010 by sinking a South Korean corvette and shelling a South Korean-populated island."
But, he added, "the threat does point to more trouble to come from the recalcitrant hermit kingdom. Things are going to get worse before they get better."
Pyongyang said the underground nuclear blast it conducted on February 12 was more powerful than its two previous detonations and used a smaller, lighter device, suggesting advances in its weapons program.
It was the first nuclear test the isolated state has carried out since its young leader inherited power in December 2011 after the death of his father, Kim Jong Il, who made building up North Korea's military strength the focus of his 17-year rule.
Like the regime's previous tests in 2006 and 2009, the move prompted widespread international condemnation, as well as a promise of tough action at the United Nations.
North Korea's government regularly rails against sanctions imposed on it.
The staging this week of joint U.S.-South Korean military exercises, known as Foal Eagle, has added to the simmering tensions, KCNA reported Monday.
It described the training exercises as "an open declaration of a war" in the face of repeated warnings from the North that they should not be held.
The exercises have "touched off the pent-up resentment of the service personnel and people of (North Korea) and compelled them to harden their pledge to take thousand-fold retaliation against the enemies," the news agency said.
But the South Korean military warned Wednesday that it would respond strongly to any attack from its northern neighbor.
"If North Korea goes ahead with provocations and threatens the lives and safety of South Koreans, our military will strongly and sternly retaliate against the command and its supporting forces," Maj. Gen. Kim Yong-hyun, a senior official at the Joint Chiefs of Staff, said at a news conference, according to the semiofficial news agency Yonhap.
Kim said South Korea notified the North that the drills with the United States "are defensive in nature."