Blinken is in Seoul for the third Summit for Democracy, an initiative of US President Joe Biden, which the South is hosting this week, and is set to meet his Korean counterpart on the sidelines for talks.

Key security allies Washington and Seoul wrapped up one of their major annual joint military training exercises last week, prompting angry retorts and tit-for-tat drills from nuclear-armed Pyongyang.

"North Korea fired an unspecified ballistic missile toward the East Sea," Joint Chiefs of Staff said, referring to the body of water also known as the Sea of Japan.

Japan also confirmed the launch, with the Japan Coast Guard saying the objects appeared to have already fallen.

The launch comes just days after the annual Freedom Shield, which this year involved double the number of troops, ended Thursday. The 11 days of joint exercises were aimed at strengthening South Korean and American deterrence against the North's nuclear and missile threats.

Pyongyang this month warned that Seoul and Washington would pay a "dear price" over the Freedom Shield drills, and later announced that North Korean leader Kim Jong Un had guided an artillery unit it says was capable of striking the South Korean capital.

The nuclear-armed North has long condemned joint US-South Korea military drills, calling them rehearsals for an invasion. It has carried out weapons tests in the past as a response to previous joint exercises of this nature.

Monday's ballistic missile test is the North's second this year, after Pyongyang launched one tipped with a manoeuvrable hypersonic warhead on January 14.

Blinken landed Sunday afternoon ahead of the democracy summit, which runs from March 18 to 20 and will bring together government officials, NGOs and civil society members.

- Boosting deterrence -

Seoul is one of Washington's key regional allies, and the United States has stationed about 27,000 American soldiers in the South to help protect it against the nuclear-armed North.

Seoul's conservative President Yoon Suk Yeol has boosted ties with Washington and sought to bury the historical hatchet with former colonial power Japan to better guard against Pyongyang's threats.

So far this year, Pyongyang has declared South Korea its "principal enemy", jettisoned agencies dedicated to reunification and outreach, and threatened war over "even 0.001 mm" of territorial infringement.

Blinken will meet South Korean Foreign Minister Cho Tae Yul, the ministry said, for discussions that will cover how to boost the alliance, as Washington and Seoul explore how to improve their so-called "extended deterrence" against North Korea.

The democracy summit has attracted some criticism due to its selective invitation list, which excludes countries that consider themselves democratic, such as Thailand and Turkey.

After Seoul, Blinken heads to the Philippines, a trip that will reaffirm "our unwavering commitment to the Philippine ally", according to State Department spokesperson Matthew Miller.

The United States is redoubling efforts to improve longstanding ties with regional allies such as Manila, in an effort to counterbalance China.