The US has urged North Korea to honour an agreement not to conduct missile tests, after it apparently fired two short-range missiles.
North Korea's nuclear ambitions have caused controversy
The US said North Korea should abide by a moratorium on missile tests which had been agreed during talks last year.
The firings on Wednesday came at a time of stalemate in negotiations aimed at resolving North Korea's nuclear crisis.
The six-nation talks on the North's nuclear ambitions began in 2003, but have yet to achieve much progress.
The short-range tests were originally reported in Asian media but later confirmed by the US State Department.
Some reports suggested the missiles were fired off accidentally during a military drill, while others say they were test-fired toward the Sea of Japan.
"We have consistently pointed out that North Korea's missile programme is a concern that poses a threat to the region and the larger international community," White House spokesman Scott McClellan said.
Correspondents say that while much of the world's attention has been focused on Iran's nuclear activities, progress over containing North Korea's nuclear ambitions has stalled.
Last September, North Korea agreed to give up its nuclear goals and return to the nuclear non-proliferation treaty.
But demands that it be given a civilian nuclear reactor - and a row over financial sanctions imposed by Washington due to alleged money-laundering and counterfeiting - have brought talks to a standstill with no date set for the next round of negotiations.