North Korea launched two short-range ballistic missiles in the direction of Japan on Thursday, Seoul and Tokyo said.
The incident comes days after North Korea fired a single intermediate-range ballistic missile over Japan on Tuesday. All three missiles landed in the sea.
"This is the sixth time in the short period just counting the ones from the end of September," Japan's Prime Minister Fumio Kishida told reporters. "This absolutely cannot be tolerated."
As he arrived at work, South Korean President Yoon Suk Yeol told reporters: "I know the people must be worried, but our government will thoroughly take care of the people's lives and safety through a strong South Korea-US alliance and security cooperation between South Korea, the US and Japan."
'Irregular' trajectory
The missiles were launched 22 minutes apart from the vicinity of Pyongyang, South Korea's Joint Chiefs of Staff said in a statement. The first missile flew 350 kilometers (215 miles) and the second one traveled 900 kilometers (500 miles).
Japanese Defense Minister Yasukazu Hamada said the second missile flew at a relatively low altitude of 60 kilometers and was possibly launched on an "irregular" trajectory.
This description has previously been used to describe a North Korean weapon modeled after Russia's Iskander missile, which travels at low altitudes and is designed to be maneuverable in flight to better evade missile defenses.
China, Russia accused of enabling Kim
The missile launch coincided with the United States accusing China and Russia of enabling North Korea's Supreme Leader Kim Jong Un.
"The DPRK (North Korea) has enjoyed blanket protection from two members of this council," US ambassador to the United Nations, Linda Thomas-Greenfield, said at a Security Council meeting.
"In short, two permanent members of the Security Council have enabled Kim Jong Un."
China's deputy UN ambassador Geng Shuang called for dialogue "instead of relying solely on strong rhetoric or pressure."
Meanwhile, Russia's deputy UN ambassador Anna Evstigneeva told the Security Council that "introducing new sanctions against the DPRK is a dead end" that would bring "zero result." She said North Korea's launch on Tuesday was prompted by the "irresponsibility" of US-led military drills in the region.
Escalating military tensions
Following North Korea's missile launch on Tuesday, South Korea and the United States retaliated with joint missile launches on Wednesday.
An hour before its additional missile launch on Thursday, North Korea's Foreign Ministry accused Seoul and Washington of "escalating the military tensions on the Korean Peninsula" by conducting the largest joint military drills in five years.
North Korea also condemned the US for repositioning its newest aircraft carrier, the USS Ronald Reagan, off the coast of South Korea.
A spokesperson for the US State Department called Thursday's missile launch a threat to the international community, but added that Washington is committed to dialogue.