SEOUL (AFP) - South Korea and the United States launched a major joint air force exercise on Friday, in a move likely to further infuriate North Korea as US Defense Secretary Ashton Carter visits Seoul.

The two-week-long "Max Thunder" drill will involve around 100 military aircraft, including jet fighters and bombers carrying out tactical bombing runs, the South Korean Airforce said.

Around 1,400 servicemen from both countries will be taking part in the drill which is held twice a year and comes against the backdrop of the annual Key Resolve and Foal Eagle joint exercises.

The drills always trigger an escalation of military tensions on the Korean peninsula, with Pyongyang denouncing them as provocative rehearsals for invasion.

Seoul and Washington insist they are purely defensive in nature.

North Korea has already underlined its anger at the drills with a series of small range rocket launches into the sea of its eastern coast.

And on Friday, as Carter arrived in Japan on the first leg of his two-nation tour of the key US military allies in Asia, the North fired two surface-to-air missiles off its west coast.

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

Carter, who arrived in South Korea on Thursday, said the missile firings were a reminder of "how dangerous things are" on the Korean peninsula and how important it was to strengthen the US-South Korea alliance.

The United States has close to 30,000 troops permanently stationed in South Korea and would assume operational command of both armed forces in the event of a conflict with the North.

The two Koreas remain technically at war because the 1950-53 Korean conflict ended with a ceasefire rather than a peace treaty.

Carter was due to hold talks in Seoul 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.

The United States is pushing to deploy a ballistic missile defence system in South Korea as a deterrent to military provocation by the North.

China has warned that deployment of the system, known as THAAD, on the Korean peninsula would undermine regional peace and stability.

A US defense official said this week that the THAAD deployment issue was unlikely to feature during Carter's talks in Seoul.

"We have got a lot of homework to do before we decide where we are going to put the THAAD batteries that are coming off the assembly line," the official said, adding that there had been no "formal discussions" with Seoul on the subject.