The incident marks the North's seventh ballistic missile launch this year and its fourth in April alone.
"As the US is focused on Iran, the North sees this as a golden time to upgrade their nuclear power and missile capability," Lim Eul-chul, a professor at Kyungnam University, said on Sunday.
South Korea's presidential office said it had held an emergency security meeting, media reports said.
Such tests violate UN Security Council resolutions against the North's missile program. Pyongyang rejects the UN ban and says it infringes its sovereign right to self-defence.
The missiles were fired near the city of Sinpo on North Korea's east coast around 6.10am on Sunday, South Korea's military said in a statement.
Japan's government posted on social media that the ballistic missiles are believed to have fallen near the east coast of the Korean Peninsula, and no incursion into Japan's exclusive economic zone has been confirmed.
The launches come as China and the United States prepare for a summit in mid-May where US President Donald Trump and Chinese President Xi Jinping are expected to discuss North Korea.
In late March, North Korean leader Kim Jong-un said Pyongyang's status as a nuclear-armed state was irreversible and expanding a "self-defensive nuclear deterrent" was essential to national security.
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