North Korea could launch new provocative action within months, a senior US military commander has said, warning of "serious consequences".
Last year Pyongyang was accused of sinking a South Korean warship and shelling a South Korean island.
Satellite images suggest the North may now have completed work on a new launch site for long-range missiles.
But Adm Robert Willard, head of the US Pacific Command, said there were no signs of an imminent launch.
In recent years satellites have captured images showing the slow development of a second, larger launchpad at a base near North Korea's border with China.
Pictures published this week by GlobalSecurity.org, a US defence information group, suggest the process is now complete.
Pyongyang conducted long-range missile tests as recently as 2009, the same year that it detonated a nuclear device underground.
Adm Willard would not comment in detail on the new satellite pictures, except to say that he knew of "no signs" North Korea was preparing for missile tests in the short-term.
Last month the US defence secretary voiced fears that Pyongyang could develop missiles which threatened the US within five years.
The more immediate concern, said Adm Willard, was what he called military provocations - referring to two incidents last year in which a South Korean warship was torpedoed, killing 46 on board, and a South Korean island was shelled, claiming the lives of four people including two civilians.
North Korea denies sinking the ship and says the shelling was provoked by South Korean military exercises.
The senior commander said there could be more such attacks within months after preliminary military talks between the two Koreas broke down this month.
Adm Willard warned that "South Korean tolerance for the next provocation appears to be very low".
Analysts suggest North Korea's strategy may be to burnish the military credentials of Kim Jong-il's youngest son, as he prepares to take over the communist state's leadership from his ailing father.
Adm Willard said there was a "compressed timeline" in which Kim Jong-il had to train his son in "coercive measures".
He confirmed that the US and South Korea would hold joint military exercises at the end of the month, which were designed as a "deterrent and a defensive measure to maintain readiness" of their combined forces.
The US has about 30,000 troops stationed in South Korea - a legacy of the 1950-53 Korean War that ended with a ceasefire rather than a formal peace treaty.