SEOUL—South Korea said it agreed with the U.S. to extend the range of its ballistic-missile systems to cover all of North Korea, going beyond the 185-mile limit of a voluntary agreement with the U.S. and other countries.

The decision comes after a year of public pressure by President Lee Myung-bak and other South Korean conservative heavyweights on the U.S. government, which formed the agreement known as the Missile Technology Control Regime in 1987 to slow the spread of missile technology. Approximately 35 countries, including South Korea, are part of the pact.

In announcing the move Sunday, Mr. Lee's national-security aide Chun Yong-woo saidSouth Korea would extend the range of its ballistic missiles to 500 miles, a distance that would mean it could hit the northeast corner of North Korea from launch sites in central South Korea. It also puts part of China's northeast and a large area of Japan within range of South Korea's missiles.

State Department and Pentagon officials said Sunday that South Korea needs the additional missile range to defend against the North Korean ballistic-missile threat.

South Korea's "new missile guidelines are designed to improve their ability to deter and defend against DPRK [North Korean] ballistic missiles," said Lt. Col. Steven Warren, a Pentagon spokesman. "These revisions are a prudent, proportional, and specific response to the DPRK ballistic-missile threat."

Harry Edwards, a State Department official, said Washington and Seoul have tried to find ways to address the North Korean ballistic threat. Leading up to Sunday's announcement, U.S. officials and South Korean officials met last month at the United Nations and at an international summit in Vladivostok, Russia.

"Based on these consultations, the Republic of Korea has announced revisions to its missile guidelines as well as additional improvements to alliance capabilities," Mr. Edwards said.

Another U.S. official described the deal as "part of our continuing effort to strengthen South Korea's capabilities against North Korea."

South Korea will limit the payload of the longer-range missiles to 500 kilograms. It will increase the payload of its 300-kilometer missiles to 1,500 kilograms.

North Korea has long had missiles that can go 310 miles, essentially covering all of South Korea.

Starting late last year, South Korean military officials and newspaper columnists began openly discussing the need for the country to extend its missile range from the limit that it agreed to in 2001 when it joined the pact.

Mr. Lee spoke publicly about modifying the South's missile-range limit in March, at a time when North Korea had just announced plans to test a long-range rocket that officials in Seoul, Washington and elsewhere believed was cover for missile technology. "An answer at the rational level will come out soon," Mr. Lee said at the time.

President Barack Obama, on a visit to Seoul in late March, said "There are no specific preconditions around, or specific obstacles around, the missile-range issue. Rather it's a broader question of what are the needs in order for us to fulfill our enduring goals around the [South Korea-U.S.] alliance."

North Korea fired its rocket in early April, its third such test of long-range missile capability. The rocket failed about a minute after takeoff.

North Korea made no immediate comment about South Korea's announcement on Sunday. The North's state media has regularly criticized Mr. Lee, including for his portrayals of the April rocket launch as a threat to South Korea.

Mr. Chun didn't announce specific plans to develop longer-range missiles. A spokeswoman for the presidential office said the military, members of Parliament and the next president are likely to take up research-and-development issues next year.

South Koreans will elect a new president in December. Mr. Lee, who is limited to one five-year term in office, will complete his presidency in February.

--Julian E. Barnes and Scott Patterson in Washington contributed to this article.