US Defense Secretary Ashton Carter arrived in South Korea on Thursday for a visit dominated by the issue of North Korea, which fired off two missiles as the Pentagon chief flew to the region.

Carter arrived from Tokyo on the second leg of a trip to the two key US military allies in Asia.

As the Pentagon chief flew to Japan on Tuesday, North Korea fired two surface-to-air missiles into the sea off its west coast -- a common, attention-grabbing tactic during high profile visits to the region.

Carter said the test firing was a reminder of "how dangerous things are" on the Korean peninsula and how important it was to strengthen the US-South Korea alliance in the face of the threat from the North.

"If it was a welcome message for me, it was flattering," Carter joked to reporters on his plane Thursday. "I've been in the job for six weeks, and that's two missiles. That's pretty good."

Although explicitly banned from doing so by UN resolutions, North Korea repeatedly carries out ballistic missile tests -- often as a means of voicing its displeasure.

It fired a series of short-range missiles into the Sea of Japan (East Sea) in March and again last week to protest annual US-South Korea military drills that Pyongyang views as rehearsals for invasion.

One of the joint drills, Key Resolve, wound up last month, but the other, Foal Eagle, is set to continue until April 24.

The annual exercises always trigger a surge in military tensions between the two Koreas, who remain technically at war because the 1950-53 Korean conflict ended with a ceasefire rather than a peace treaty.

Carter was scheduled to hold talks Friday with South Korean President Park Geun-Hye, as well as her national security adviser and defence minister.

He will also visit a memorial to the 46 seamen who died in the 2010 sinking of the South Korean naval corvette Cheonan.

A South Korean-led investigation involving a team of international experts concluded the ship was sunk by a North Korean submarine torpedo, but Pyongyang has always denied involvement.

During his trip, Carter is expected to underline Washington's continued commitment to the military alliance with South Korea, and to discuss the eventual transfer of the Pentagon's command control of Korean troops.

In the event of war with North Korea, current plans call for an American military commander to lead the 28,500 US troops deployed in South Korea as well as that country's 640,000-strong force.

Full operational control was to have switched to South Korea in 2015 but Seoul asked to postpone the transition.