A more likely scenario, they say, is a localized provocative move.
Amid the fiery words from Pyongyang and annual military training exercises by U.S. and South Korean forces in the region, government officials in Washington and Seoul say they are taking the North Korean threat seriously.
In the days before North Korea's latest round of threats and provocations, U.S. and North Korean officials met in New York, although nothing came of the meeting, said a source familiar with what happened.
The source described the meeting as part of regular back channel exchanges between the countries.
Clifford Hart, the U.S. envoy for six-party talks aimed at North Korean denuclearization, met with North Korea's Deputy UN Ambassador Han Song-ryol in mid-March, according to the source.
Hart repeated the Obama administration's call for North Korea to avoid provocative actions and urged a return to diplomacy. Han promised to carry the messages back to Pyongyang, the source said.
The North was blamed for two attacks on South Korea in 2010, one on a navy vessel and another on the island of Yeonpyeong. Those attacks killed 50 people. Pyongyang still denies responsibility for the sinking of the South Korean warship, the Cheonan, in which 46 sailors died.
On Tuesday, Japan said it had deployed missile defense systems around Tokyo amid expectations that the North could carry out a missile test in the coming days.
The Japanese government is making "every possible effort to protect the Japanese people and ensure their safety," said Prime Minister Shinzo Abe.
South Korean government officials have said they think North Korea could conduct the test launch of a missile as soon as Wednesday, following reports that the North had loaded as many as two medium-range missiles onto mobile launchers on its east coast.
The United States had previously said it was moving missile defense systems to Guam, a Western Pacific territory that is home to U.S. naval and air bases. North Korea has cited those bases when listing possible targets for missile attacks.
A symbol of cooperation at risk
The souring situation on the Korean Peninsula was in evidence in the failure of more than 50,000 North Korean workers to show up for work Tuesday morning at the Kaesong Industrial Complex, the manufacturing zone shared by the two Koreas that had operated without such an interruption for eight years.
The North had declared Monday that it would pull out its workers and temporarily suspend activities at the complex, which sits on its side of the heavily fortified border but houses the operations of more than 120 South Korean companies.
On Tuesday, the South Korean Unification Ministry said the North Korean workers hadn't reported for work in the district, which is the last major symbol of cooperation between the two Koreas.
Analysts had expressed skepticism that Pyongyang would follow through on previous threats to shut down the complex, noting that it is an important source of hard currency to Kim's regime.
The move also is likely to put pressure on the city of Kaesong itself, where the North Korean workers and their families live. With an estimated population of between 200,000 and 300,000 people, it is one of the impoverished country's largest cities.