North Korea fired two more ballistic missiles on Sunday, marking the seventh launch in recent days amid tensions over the U.S.-South Korea drill.

Both missiles reached an altitude of 100 kilometers (60 miles) and covered a range of 350 kilometers, Japan's State Minister of Defense Toshiro Ino told reporters. The first was fired at around 1:47 a.m. local time (4:47 p.m. GMT) and the second one some six minutes later.

Both fell outside Japan's exclusive economic zone and authorities were looking into what kind of missiles were launched, including the possibility that they were submarine-launched ballistic missiles, he said.

On Tuesday, nuclear-armed North Korea test-fired a ballistic missile farther than ever before, sending it soaring over Japan for the first time in five years and prompting a warning for residents there to take cover.

Ino said Tokyo would not tolerate the repeated actions by North Korea. The incident marks the seventh such launch since Sept. 25.

North Korea, which has pursued missile and nuclear tests in defiance of U.N. sanctions, said on Saturday its missile tests were for self-defense against direct U.S. military threats and had not harmed the safety of neighboring countries and regions.

"Our missile tests are a normal, planned self-defense measure to protect our country's security and regional peace from direct U.S. military threats," said state media KCNA, citing an aviation administration spokesperson.

The United States and South Korea held joint maritime exercises on Friday, a day after Seoul scrambled fighter jets in reaction to an apparent North Korean bombing drill.

The United States also announced new sanctions on Friday in response to North Korea's latest missile launches.