Pyongyang also fired similar weapons minutes before South Korean President Lee Jae Myung held talks with Chinese leader Xi Jinping last week, the South Korean Joint Chiefs of Staff (JCS) said.

The JCS said they had recently "detected about 10 artillery rockets fired into the northern part of the West Sea", Seoul's name for the Yellow Sea.

The weapons were fired at around 3:00 pm (0600 GMT) on Saturday and around 4:00 pm on Monday.

"Details of the projectiles are currently being closely analysed by South Korean and US intelligence authorities," the JCS added.

Hegseth visited the heavily fortified border dividing North and South Korea on Monday, becoming the first Pentagon chief in eight years to do so.

He toured Panmunjom, the symbolic truce village where troops from both Koreas stand face-to-face, following a stop at Observation Post Ouellette overlooking the Demilitarized Zone.

Hegseth and South Korean counterpart Ahn Gyu-back "reaffirmed the strong combined defence posture and close cooperation between South Korea and the United States", Seoul's defence ministry said in a statement.

Hegseth said at a joint news conference with Ahn on Tuesday that South Korea faces a "dangerous security environment" and the two ministers agreed to remain "clear-eyed about the threats" they face.

He also said South Korea's increase in defence spending would accelerate its "ability to lead its conventional deterrence and defence against North Korea".

President Lee said Tuesday Seoul would make its biggest defence budget increase in six years with an 8.2 percent rise from this year to 66.3 trillion won ($4.6 billion).

Hegseth's trip comes after US President Donald Trump's overtures to North Korean leader Kim Jong Un during his Asia tour last week drew no public response from Pyongyang.

However, Trump has indicated that he would still be willing to "come back" for a future meeting with Kim.

On Saturday, Lee met Xi on the sidelines of an Asian economic summit, urging the Chinese leader to help Seoul "resume dialogue" with North Korea.

Lee stressed the need for regional "stability" and noted "recent high-level exchanges between China and North Korea" -- a reference to Kim's attendance at a major military parade in Beijing in September.