North Korea puts rockets on standby to "settle accounts" with U.S. amid fears isolated regime may lash out

Updated at 7:20 a.m. Eastern.

SEOUL, South Korea North Korean leader Kim Jong Un warned Friday that his rocket forces were ready "to settle accounts with the U.S.," unleashing a new round of bellicose rhetoric after U.S. nuclear-capable B-2 bombers dropped dummy munitions in joint military drills with South Korea.

Kim's warning, and the litany of threats that have preceded it, don't indicate an imminent war. In fact, they're most likely meant to coerce South Korea into softening its policies, win direct talks and aid from Washington, and strengthen the young leader's credentials and image at home.

But the threats from North Korea and rising animosity from the rivals that have followed U.N. sanctions over Pyongyang's Feb. 12 nuclear test do raise worries of a misjudgment leading to a clash.

CBS News correspondent Margaret Brennan says top U.S. officials have classified this as a "serious provocation phase" by Pyongyang, and concerns are focused on the prospect of North Korea firing on South Korea, drawing them -- and possibly the U.S. -- into a cross-border conflict.

Kim "convened an urgent operation meeting" of senior generals just after midnight, signed a rocket preparation plan and ordered his forces on standby to strike the U.S. mainland, South Korea, Guam and Hawaii, state media reported.

Kim said "the time has come to settle accounts with the U.S. imperialists in view of the prevailing situation," according to a report by the North's official Korean Central News Agency.

South Korea's Yonhap news agency quoted military officials in the country as saying there had been "increased activity" at some of the North's missile sites in recent days.

"North Korea's launch sites to fire off mid- and long-range missiles have recently shown increased movement of vehicles and forces," Yonhap quoted one anonymous official as saying. "We are closely watching possibilities of missile launches."

Brennan notes that North Korea is so isolated, there's little intelligence to confirm exactly what weapons are in the regime's arsenal.

Experts believe the country is years away from developing nuclear-tipped missiles that could strike the United States. Many say they've also seen no evidence that Pyongyang has long-range missiles that can hit the U.S. mainland.

Still, there are fears of a localized conflict, such as a naval skirmish in disputed Yellow Sea waters. Such naval clashes have happened three times since 1999. There's also the danger that such a clash could escalate. Seoul has vowed to hit back hard the next time it is attacked.

North Korea's threats are also worrisome because of its arsenal of short- and mid-range missiles that can hit targets in South Korea and Japan. Seoul is only a short drive from the heavily armed border separating the Koreas.

"The North can fire 500,000 rounds of artillery on Seoul in the first hour of a conflict," analysts Victor Cha and David Kang wrote recently for Foreign Policy magazine. They also note that North Korea has a history of testing new South Korean leaders; President Park Geun-hye took office late last month. "Since 1992, the North has welcomed these five new leaders by disturbing the peace," they wrote.

Behind the scenes, Brennan says American diplomats are trying to get the Japanese, Chinese and South Koreans to coordinate their response in the event that North Korea opens fire. Secretary of State John Kerry will visit all three countries next month to try to de-escalate the tension.

In the past, China has been able to pressure the North to reign in its rhetoric by threatening to cut off financial support. But this time, North Korea's young president hasn't responded to Beijing's demands that he stop all aggression.

Later Friday at the main square in Pyongyang, tens of thousands of North Koreans turned out for a 90-minute mass rally in support of Kim's call to arms. Men and women, many of them in olive drab uniforms, stood in arrow-straight lines, fists raised as they chanted, "Death to the U.S. imperialists." Placards in the plaza bore harsh words for South Korea as well, including, "Let's rip the puppet traitors to death!"

North Koreans march through Kim Il Sung Square
North Koreans march through Kim Il Sung Square in downtown Pyongyang, North Korea, March 29, 2013. Soldiers and students are shown holding placards that read: