North Korea has taken a major step back from a planned missile test, US officials said, even as Pyongyang and Seoul exchanged fresh threats Tuesday of swift military retaliation to any provocation.

A US defence official said two North Korean missiles -- primed for imminent test firing -- had been moved from their launch site, signalling a possible easing of North Asia tensions ahead of a US-South Korea summit in Washington.

US and South Korean officials had been worried that any test of the medium-range Musudan missiles would trigger a fresh surge in military tensions that have included threats of nuclear war from Pyongyang.

But the US defence official told AFP on condition of anonymity: "They moved them," and added that there was no longer an imminent threat of a test.

Pyongyang, which rattled the world with its third nuclear test in February, would have to make detectable preparations to return to a launch-ready status, two US officials said.

The move was revealed in Washington on the eve of a first summit between President Barack Obama and new South Korean President Park Geun-Hye at the White House on Tuesday, intended as a strong signal of unity to Pyongyang.

Tensions on the Korean peninsula have been on the brink of boiling over for months, with the North issuing a series of apocalyptic threats over what it sees as intensely provocative US-South Korean military exercises.

Although large-scale, annual joint drills were wrapped up at the end of last month, Pyongyang issued a fresh warning Tuesday over a smaller, joint anti-submarine exercise.

North Korean troops near the disputed Yellow Sea border with the South sector have been ordered to strike back if "even a single shell drops" in their territorial waters, the North's army command said in a statement.

Any subsequent counterstrike would trigger an escalated military reaction that would see South Korea's border islands engulfed in a "sea of flames," the statement added.

North Korea shelled one of the islands, Yeonpyeong, in November 2010, killing four South Koreans and sparking brief fears of a full-scale conflict.

In an interview with US broadcaster CBS ahead of her summit with Obama, President Park said any similar attack by the North would be met with a harsh military response.

"Yes, we will make them pay," Park said, adding that Seoul would no longer engage in a "vicious cycle" of automatically meeting the North's provocations and threats with negotiations and assistance.

"It is time for us to put an end to that cycle," she said.

Earlier, a senior White House official warned that it was too early to say whether North Korea's spate of bellicose behaviour, which prompted Washington to send nuclear-capable stealth B-2 bombers over South Korea, was ending.

"It's premature to make a judgment about whether the North Korean provocation cycle is going up, down or zigzagging," said Danny Russel, senior director for East Asia on Obama's National Security Council.

South Korea's defence ministry would not comment on whether the North had moved its Musudan missiles from their launch pads, saying only that it was closely "tracking" all activity in the area.

It also declined to confirm a Yonhap news agency report, citing a senior government source in Seoul, that North Korea had ordered missile and rocket units to stand down from combat-ready status.

Washington is making strenuous efforts to cement Obama's relationship with Park who was sworn into office less than three months ago.

Obama will host South Korea's first woman leader in the Oval Office, hold an expanded luncheon meeting for both delegations, then appear with Park at a joint White House press conference.

Park will address a joint session of the US Congress on Wednesday.

Park has taken a firm stand against any concessions to North Korea but has also been careful not to close the door to future talks -- which US officials say is ultimately the sole, albeit not ideal, way to deal with Pyongyang.

Russel said the theatrics of the White House summit were intended to send a clear signal to Pyongyang.

"The unity of message between the two governments and the two presidents signals to North Korea that it has no hope of gaining benefits from provocation," he said.

burs-jhw/gh/mtp