Citing North Korea's suggestion that diplomats leave the country, South Korean President Park Geun-hye's national security director said the North may be planning a missile launch or another provocation around Wednesday, according to presidential spokeswoman Kim Haing.
In Washington, an adviser to President Barack Obama said "we wouldn't be surprised if they did a test. They've done that in the past."
Aide Dan Pfeiffer told ABC's "This Week" that "the key here is for the North Koreans to stop their actions, start meeting their international obligations, and put themselves in a position where they can achieve what is their stated goal, which is economic development, which will only happen if they rejoin the international community."
He told "Fox News Sunday" that "the onus is on the North Koreans to do the right thing here," adding that "they are the source of the problem and the only way to solve this is for them to take a step back."
If they don't, there will be consequences, Pfeiffer said.
"They will be able to further isolate themselves in the world, they will continue to further hurt themselves. The North Korean people are starving because of actions like the ones North Koreans are taking right now."
U.S. Sen. John McCain said the North's young leader, Kim Jong Un, is playing a game of brinksmanship.
"In the past we have seen this repetitious confrontation, negotiation, incentives to North Korea to better behave, hopes that they will abandon their nuclear quest — which they never will, otherwise, they'd be totally irrelevant," McCain told CBS' "Face the Nation."
"And so we've seen the cycle over and over and over again, for last 20 or 30 years. They confront. There's crisis. Then we offer them incentives — food, money. While meanwhile the most repressive and oppressive regime on earth continues to function," he added.
McCain said China "does hold the key to this problem. China can cut off their economy if they want to."
Associated Press writers Lolita C. Baldor, Philip Elliott and Erica Werner in Washington contributed to this report.