The US military in South Korea has asked the Pentagon to provide more attack helicopters and strengthen missile defence systems, its chief said Tuesday, amid threats by North Korea against the South.

"In order to enhance war-fighting capabilities, I have asked for prioritisation to receive an additional attack and reconnaissance squadron to bring our combat aviation brigade to full strength," General James Thurman told a forum.

"And I have asked for increased capabilities in terms of theatre ballistic missile defence," said Thurman, adding he is confident the request will be met.

His comments come at a time of high cross-border tension, with the North threatening "sacred war" against the South following perceived insults to its regime.

On June 4 the North's military threatened attacks on the Seoul headquarters of major South Korean media outlets after they criticised a mass youth function staged in Pyongyang.

The US fought for the South in the 1950-53 war against the North and China, and has based troops in the South ever since. They currently total 28,500.

"I'm confident we will be able to work this," Thurman said, adding his top priority was to maintain a "stable and peaceful" situation on the Korean peninsula.

"I will ensure that we maintain the highest level of readiness," he added.

The North has struck a consistently hostile note with the South since new leader Kim Jong-Un took over following the death of his father Kim Jong-Il last December.

Thurman, in comments to the US House of Representatives' Armed Services Committee in March, spoke of a "very uncertain period" on the peninsula, "with the possibility of unexpected events leading to miscalculation".

There is widespread expectation the North will conduct a nuclear test following United Nations censure of its failed rocket launch in April, although Pyongyang said Saturday it has no plans "at present" to do so.

Some Seoul analysts also believe the North may engineer a border clash.

On Monday the South's military held an unscheduled readiness exercise to check its defence posture, and warned it would immediately retaliate against "core command forces" if attacked by the North.

Seoul accuses Pyongyang of torpedoing a warship in March 2010 with the loss of 46 lives. The North denied the charge but killed four South Koreans when it shelled a border island in November 2010.