All eyes are on North Korea on Monday to see if it marks the birthday of late founder Kim Il-Sung with an expected missile launch, despite tension-reducing noises from Seoul and Washington.

North Korea has a habit of linking high-profile military tests with key dates in its annual calendar. The centenary of Kim's birth last year was preceded by a long-range rocket test that ended in failure.

South Korean intelligence says the North has had two medium-range missiles primed and ready to fire for nearly a week, with many observers tapping Monday's anniversary as a likely launch date.

US Secretary of State John Kerry, currently in Japan on the last leg of a whirlwind Northeast Asia tour, warned North Korea when he was in Seoul on Friday that a launch in the current climate would be a "huge mistake".

The Korean peninsula has been in a state of heightened military tension since the North carried out its third nuclear test in February.

Incensed by fresh UN sanctions and joint South Korea-US military exercises, Pyongyang has spent weeks issuing blistering threats of missile strikes and nuclear war.

During his visits in Seoul, Beijing and Tokyo, Kerry talked tough on the North's "unacceptable" rhetoric, but also sought to lower the temperature slightly by supporting a dialogue with Pyongyang and saying he would be prepared to reach out to North Korea.

He also urged North Korean leader Kim Jong-Un to lead his country back to negotiations.

"We're prepared to reach out, but we need the appropriate moment, appropriate circumstances," Kerry said Sunday.

In Seoul, he gave Washington's public blessing to peace overtures made by South Korea's new president, Park Geun-Hye, who in recent days has signalled the need to open a dialogue and "listen to what North Korea thinks".

Park had campaigned on a promise of greater engagement with Pyongyang, but had her hands tied by the international outcry over the North's nuclear test and the subsequent sharp escalation of tensions.

The North's immediate response to her latest remarks was negative, with a spokesman for the North's Committee for the Peaceful Reunification of Korea suggesting the dialogue overtures were a "cunning" ploy.

"We found the offer an empty, meaningless act," the spokesman said in an interview Sunday with the state media KCNA.

"If the South is genuine about having talks...it should first abandon its confrontational posture," he said, citing ongoing military drills with the US.

Monday's celebrations in Pyongyang will have their usual martial flavour, with a large military parade that North Korea uses to showcase its weaponry to the world.

The missiles mobilised by the North are reported to be untested Musudan models with an estimated range of up to 2,485 miles (4,000 kilometres).

That would cover any target in South Korea and Japan, and possibly even US military bases on the Pacific island of Guam.

South Korean and US forces have been on a heightened state of alert for days, and Japan has deployed Patriot anti-missile systems around Tokyo and promised to shoot down any missile deemed to be a threat.

In Beijing on Saturday, Kerry had pressed Chinese leaders to take a firmer stand with North Korea.

China is Pyongyang's sole major ally and backer, and is widely seen as the only country with leverage to influence its actions -- although it is reluctant to risk destabilising the regime.

Kerry won a promise that China would work together with the United States to reduce tensions and persuade the North to give up its nuclear weapons programme.

The ball now lies very much in Pyongyang's court in terms of pushing ahead with a missile launch or not, and observers note that the North's response to diplomatic pressure in the past has often been a provocative show of force.