The U.S. military conducted a successful test of its Terminal High Altitude Area Defense (THAAD) missile system on Tuesday by shooting down a simulated intermediate-range ballistic missile over the Pacific Ocean. The test signals an improvement in the defense system, which before Tuesday had only intercepted shorter-range missiles. While the simulation was planned for months, it comes amid a growing international threat from North Korea, which tested its first intercontinental ballistic missile (ICBM) on July 4.

On Tuesday, the U.S. Missile Defense Agency released a statement claiming that the latest test “bolsters the country’s defensive capability against developing missile threats in North Korea and other countries around the globe.” North Korea is widely believed to be developing an ICBM capable of reaching the U.S. mainland. Experts say the nation’s latest tests puts Alaska within striking distance, but is not yet capable of reaching the lower 48 states.

Tuesday’s test was carried out in Kodiak, Alaska, and shot down a ballistic missile target from an aircraft flying north of Hawaii. The simulation adds to a perfect track record of THAAD missile launches since the U.S. resumed testing in 2005. Thus far, THAAD has proven more successful than the nation’s Ground-based Midcourse Defense (GMD) system, which is designed to target an ICBM. Even after a successful test launch in May, the GMD system has only achieved a 55 percent success rate. While the Pentagon previously reported that the system has a “limited capability to defend the U.S. homeland,” they have since claimed that the U.S. is capable of defending itself against a small number of ICBMs.

Unlike the GMD system, the THAAD system is designed to counter short, medium, and intermediate range missile threats. In June, the U.S. Missile Defense Agency announced plans to deliver an additional 52 THAAD interceptors to the U.S. Army between October 2017 and September 2018. The nation has also deployed the system in South Korea and Guam—areas that both contain U.S. military bases. The deployment of THAAD in South Korea has sparked criticism from nations like Russia and China, who accuse the U.S. of attempting to expand its military authority in Asia. North Korea has also called for an end to the program, which it considers a threat to regional stability.

In May, South Korea’s president, Moon Jae In, ordered an investigation into the nation’s THAAD system, placing the project on shaky ground. Moon has previously advocated for more diplomatic relations with North Korea that include increased dialogue. By July, however, Moon told former President Obama that North Korea was approaching its “last opportunity” to engage in peaceful negotiations. The day after North Korea launched an ICBM, the U.S. and South Korea responded with a joint missile drill.

Despite North Korea’s growing threat to international safety, Russia and China have continually advocated for a mutual exchange in which North Korea, South Korea, and the U.S. each agree to suspend their testing. Thus far, the U.S. has expressed little interest in this strategy, instead relying on China, North Korea’s only major ally, to convince the nation to renounce its nuclear and missile programs. While China has indicated a willingness to negotiate with North Korea, the nation has become increasingly frustrated by mounting pressure from the U.S. At a Tuesday news briefing, the nation’s foreign ministry spokesman, Shuang Geng, argued that “China is not to be blamed for the current escalation of tension, nor does China hold the key to resolve the issue.” Curbing the North Korean threat was impossible, he added, when “China is striving to put out the fire, while the others are fueling the flame.”