South Korea's Joint Chiefs of Staff reported the detection of "several" suspected short-range ballistic missile launches by North Korea on Monday morning.
Japanese Prime Minister Fumio Kishida told a parliamentary session that the North Korean missiles landed in the waters between the Korean Peninsula and Japan.
No damage or injuries have been reported.
The US State Department criticized the missile launches, calling them a threat to the security of the region.
Blinken attending democracy summit
This latest military posturing by Pyongyang comes as Blinken was visiting Seoul to attend a conference hosted by South Korean President Yoon Suk Yeol.
Blinken is also scheduled to hold talks with his South Korean counterpart on the sidelines of the third Summit for Democracy in Seoul.
The summit is an initiative of US President Joe Biden, focusing on exploring strategies to stop the global decline of democracy and the erosion of rights and freedoms.
The meeting, running from March 18 to 20, will host government officials, NGOs and civil society members.
The US secretary of state's trip includes meetings with South Korean officials, and aims to reinforce alliances.
US-South Korea conclude joint military exercise
The North Korean missile launch comes just after the 11-day annual Freedom Shield joint military exercises between the US and South Korea concluded on Thursday.
North Korea has condemned the exercises in the past as preparation for invasion.
Pyongyang recently threatened Seoul and Washington with severe consequences for their Freedom Shield exercises, saying there would be a "dear price" to be paid.
In January, tensions soared after North Korean leader Kim Jong Un proposed constitutional changes to abandon peaceful Korean unification efforts.
Kim suggested declaring South Korea as its "invariable principal enemy," and threatened annexation in case of war.
Tensions in the peninsula have been high with North Korea's frequent missile tests since 2022.
