* Submarine launch an escalation before key diplomatic events
* N.Korea may conduct nuclear test as soon as this month-officials
* S.Korea to seek deterrence against North's nuclear threat-aide (Adds comment from U.S. State Department)
By Joyce Lee and Kantaro Komiya
SEOUL/TOKYO, May 7 (Reuters) - North Korea fired a ballistic missile from a submarine on Saturday, South Korea said, an escalation just before the inauguration of a South Korean president who has vowed to take a hard line against the North and the visit of the U.S. president.
South Korean military said North Korea fired what is believed to be a submarine-launched ballistic missile (SLBM) into the sea off its east coast around 0507 GMT on Saturday from near Sinpo, where North Korea keeps submarines as well as equipment for test-firing SLBMs.
Japan also said the projectile was a short-range ballistic missile. Defence Minister Nobuo Kishi said North Korea's recent development in nuclear missile-related technology and repeated launches of ballistic missiles threatened the region and the international community.
"This is absolutely unacceptable," he told reporters, adding that Japan will continue to "strengthen defence capabilities drastically" to protect its citizens from such security threats, in close cooperation with the United States, South Korea and other allies.
The launch comes three days before Tuesday's inauguration of Yoon Suk-yeol as South Korea's president, and ahead of his May 21 summit with U.S. President Joe Biden in Seoul.
South Korea's National Intelligence Service chief Park Jie-won said North Korea may conduct a nuclear test between the inauguration and the Biden visit, Yonhap news agency reported.
Kishi said it is possible for North Korea to complete nuclear test preparations as early as this month, and take further provocative acts.
This was also in line with a U.S. assessment that Pyongyang was preparing its Punggye-ri nuclear test site and could be ready to conduct a test there as early as this month.
"This is aiming at the (South's) new administration beginning next week, and applying preemptive pressure to take control of the situation before the U.S.-South Korea summit," said Yang Moo-jin, a professor at the University of North Korean Studies in Seoul.
"It also creates tension to strengthen the regime's internal coherence in the face of circumstances such as prevention of COVID-19 spreading."
The United States condemned the launch as a threat to North Korea's neighbors and the world. A spokesperson for the U.S. State Department said Washington's commitment to defending South Korea and Japan "remains ironclad."