"It is an unfortunate and continuing example of provocation by the North Koreans," Clinton told a news briefing at a conference in The Hague, Netherlands.
Nuclear-armed North Korea warned Japan on Tuesday that intervening in Pyongyang's impending rocket launch would be considered an act of war.
North Korea says it will send a communications satellite into orbit on a multi-stage rocket between April 4 and 8. The U.S., South Korea and Japan think the communist regime is using the launch to test long-range missile technology, and they warn Pyongyang would face sanctions under a U.N. Security Council resolution banning the country from ballistic activity.
Japan has deployed battleships and Patriot missile interceptors off its northern coast to shoot down any rocket debris that the North has said might fall over the area.
Tokyo has said it is only protecting its territory and has no intention of trying to shoot down the rocket itself, but North Korea said it is not convinced and accused Japan of inciting militarism at home to justify developing a nuclear weapons program of its own.
If Japan tries to intercept the satellite, the North's army "will consider this as the start of Japan's war of re-invasion more than six decades after the Second World War and mercilessly destroy all its interceptor means and citadels with the most powerful military means," the North's official Korean Central News Agency said Tuesday.
China, North Korea's neighbor and often-estranged ally, continued to appeal for all the powers in the region to show restraint and "refrain from any action that would further complicate the situation," Chinese Foreign Ministry spokesman Qin Gang told reporters in Beijing.
But Japanese Prime Minister Taro Aso said he is ready to pursue punishment by the Security Council if North Korea fires the rocket, which already is on the launch pad.
"It would be crucial for the international community to make concerted action," Aso told a news conference after both houses of Japan's parliament passed a resolution strongly urging the North to forego the launch.
Daniel Pinkston -- a Seoul-based expert for the International Crisis Group think tank, which provides detailed analysis about North Korea -- said the communist nation has two underground nuclear warhead storage facilities near bases for its medium-range Rodong missiles, which are capable of striking Japan. The North is believed to have five to eight warheads, he said.
But he stressed it is unclear if the communist nation has mastered the technology necessary to miniaturize the warheads and put them on Rodong missiles, which have a range of 620 to 930 miles (1,000 to 1,500 kilometers).
The National Intelligence Service, South Korea's main spy agency, said it could not confirm Pinkston's claims.
Pinkston said he obtained the information from intelligence officials from a country or countries that he wouldn't identify.
"Their assessment is that North Korea has deployed" and assembled "nuclear warheads for Rodong missiles," Pinkston told The Associated Press.
Kim Tae-woo, a missile expert at the state-run Korea Institute for Defense Analyses in Seoul, said the North has been focusing on efforts to mount nuclear warheads on the Rodongs because the long-range Taepodong series has not been fully deployed yet.
"Rodong is the most likely weapon to be mounted with nuclear warheads," Kim said. He said it's also "natural" for the North to try to put a nuclear warhead on a missile with a longer range.
Two U.S. destroyers are believed to have departed from South Korea to monitor the rocket launch. South Korea is also dispatching its Aegis-equipped destroyer, according to a Seoul military official who asked not to be named, citing department policy.
North Korea claimed Tuesday that the U.S. and South Korea have conducted about 190 spy flights over its territory in March, including over the sea off the launch site on its northeast coast.
Further fueling tensions, hundreds of U.S. and South Korean troops conducted an air assault exercise Tuesday that the two countries have claimed is unrelated to the rocket launch. Pyongyang has strongly condemned similar joint drills in the South as preparations to invade the North.
The two allies conducted large-scale annual exercises for 12 days in March, prompting angry reaction from Pyongyang, including threats to South Korean passenger planes and repeated halts in cross-border traffic.
Adding to the complexity of the situation, the North announced Tuesday it will indict and try two American journalists accused of crossing the border illegally from China on March 17 and engaging in "hostile acts."
The North may try to use the detentions as a bargaining tool after the rocket launch, said Yang Moo-jin, an analyst at Seoul's University of North Korean Studies.
The Associated Press contributed to this report.