Japan is considering adding the U.S. Terminal High Altitude Area Defense (THAAD) system as a third layer to its ballistic-missile defenses, according to an official leak evidently prompted by North Korea's July 4 tests of R-17 and Rodong rockets.

Three or four installations of the Lockheed Martin THAAD system could cover all of Japan, says the Mainichi newspaper in an unattributed report presumed to be based on government-leaked information.

THAAD's range is secret, but the Mainichi's sources say it can reach "more than 100 kilometers (60 miles)," compared with about 20 km for the Lockheed Martin PAC-3 system that Japan currently uses as the second of its two defense layers. The first is based on Raytheon SM-3 missiles loaded on four Kongou-class destroyers fitted with the Aegis Ballistic Missile Defense system promulgated in the U.S. Navy.

North Korea tried a longer-ranged version of the old Soviet-era R-17 (Scud) missile on July 4, according to a South Korean report. The so-called Scud ER has a range of 1,000 km, enough to reach much of Japan, says the Chosun Ilbo newspaper, also close to government officials. That range is notably longer than the 750-800 km previously attributed to the Scud ER.

Three of the seven rockets that North Korea launched were Scud ERs, two were other R-17s and three were Rodongs, which are larger and longer-ranged than R-17s.

The importance of the 1,000-km range attributed to the Scud ER may be that such a rocket would be a much smaller and cheaper weapon with which to menace Japan than the Rodong would be. Most North Korean ballistic missiles are R-17 derivatives, reflecting their cheapness and the country's familiarity with making them.

The launches violated U.N. Security Council resolutions.

They also showed greater accuracy, according to the South Korean defense ministry. "We are aware that accuracy has been improved," says the ministry.

Previous launches had shown large errors, but "much improvement has been made this time in that regard," the ministry says.

The result is that North Korea can cause serious damage to specific targets in the south, including command centers and runways, with conventional explosive warheads, says an official quoted by another South Korean newspaper, the Dong-A Ilbo.

The same threat ostensibly applies to Japan, too.