The North's April launch, which is estimated to have sent a rocket about 2,000 miles (3,200 kilometers), represented a "significant advance" in the country's long-range rocket technology, the Bulletin of Atomic Scientists said in a recent report.

South Korea believes the Taepodong 2 can travel at least 4,100 miles (6,700 kilometers), putting Alaska and Guam within striking distance. The North is also believed to be developing an advanced version of the Taepodong 2 that could reach not only Hawaii, but also the West Coast of the U.S. with a potential range of about 5,000 miles (8,000 kilometers).

Pyongyang had earlier marked a large area of water off its east coast as a no-sail zone through July 10, citing military drills. Thursday's launches of four short-range missiles were believed to be the North's first military action in the designated zone.

Yonhap news agency, citing an unnamed military official, reported that all four missiles flew about 60 miles (100 kilometers) and identified them as KN-01 missiles with a range of up to 100 miles (160 kilometers).

Japanese Prime Minister Taro Aso denounced the launches as "provocative." South Korea's foreign minister, Yu Myung-hwan, said the firings are "not a good sign because they are demonstrating their military power."

South Korean analysts were skeptical about the possibility of a long-range launch anytime soon.

Koh Yu-hwan, a professor at Seoul's Dongguk University, said he expects the North will take more time to assess international reaction to its recent pledge to expand its nuclear program.

Tensions over North Korea's actions come as its leader Kim Jong Il has reportedly been laying the groundwork to hand power over to one of his sons, and as two American journalists were imprisoned for illegal border crossing and hostile acts.

Analysts predict the North will continue its provocative acts in an attempt to command world attention that can lead to economic benefits.

"I think what North Korea will continue to do is ratchet up the tension," said Brad Glosserman, another analyst at the CSIS think tank. "It needs that attention to get the concessions from other countries ... as well as to demonstrate its strength to domestic constituencies."

President Barack Obama has vowed the U.S. won't make the same mistake of rewarding North Korea's bad behavior, and his administration has been pressing China — a key North Korean ally — to enforce the new U.N. sanctions against Pyongyang.

In an interview with The Associated Press Thursday, Obama said he was trying to "keep a door open" for North Korea to return to international nuclear disarmament talks, but the country must abandon its nuclear weapons programs before it can join the world community.

Philip Goldberg, in charge of coordinating the implementation of sanctions against the North, told reporters in Beijing that he had "very good conversations" with Chinese officials Thursday, though did not give details of the talks .

Associated Press writers Kwang-tae Kim, Hyung-jin Kim and Kelly Olsen in Seoul, Alexa Olesen in Beijing, Mari Yamaguchi in Tokyo, Mike Eckel in Moscow and Pamela Hess in Washington contributed to this report.