North Korea fired a projectile into the Sea of Japan east of North Korea early Monday, apparently protesting last week’s U.S.-South Korea joint military drills, the South Korean military said.

Yonhap News Agency, quoting a military official at the Defense Ministry, said the projectile might have been an intercontinental ballistic missile that could reach the U.S. mainland.

The Joint Chiefs said in a text message the projectile was launched from the Dongchang-ri long range missile site area at 7:30 a.m. (5:30 p.m. EST Sunday), the second test since U.S. President Donald Trump took office. It flew across the country before splashing down.

North Korea has been conducting a series of missile tests with ever-increasing range.

The North threatened to take “strong retaliatory measures” after the joint exercises began Wednesday. The drills test readiness against a possible attack by the North, but Pyongyang has characterized the annual drills as preparations for war against it.

The North claimed a missile launched last month was capable of carrying a nuclear warhead. That missile traveled 300 miles before plunging into the Sea of Japan.

The New York Times reported Sunday the U.S. has been waging cyberattacks on the North’s missile program to delay development of an ICBM for the past three years. The attacks involved fiddling with the rockets’ programming so they would veer off course and explode in flight.

In the past eight months, Pyongyang has managed to launch three medium-range missiles successfully.

The Pentagon has developed an anti-missile system that involves multistage rockets launched from underground silos at Vandenberg Air Force Base in California and Fort Greely, Alaska, with a third site on the East Coast under consideration.

The Ground-based Midcourse Defense system is expected to have 44 interceptors by the end of the year but problems with the circuit boards on some of the older rockets have led to questions about the system’s reliability.