South Korea voiced regret Monday at North Korea's dismissal of its offer for dialogue, as the South's armed forces remained on heightened alert for an expected missile test by Pyongyang.

Both President Park Geun-Hye and her Unification Ministry have, in recent days, made tentative proposals for talks in an apparent bid to reduce soaring military tensions on the Korean peninsula.

The North's immediate response was negative, calling the offers "empty" and a "crafty trick" to cover up Seoul's aggressive and confrontational policies.

"It is very regrettable that the North dismissed our offer for dialogue, which had been made after much deliberation," Unification Ministry spokesman Kim Hyung-Seok told reporters.

The response from Pyongyang was "totally incomprehensible to us and the international community," he added.

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.

US Secretary of State John Kerry, currently in Tokyo on the last leg of a whirlwind tour to South Korea, China and Japan, has backed the South's talks proposal and indicated that the US also wanted a return to dialogue.

North Korea celebrated Monday the birthday of late founder Kim Il-Sung, as the world watched to see if it would push ahead with an expected medium-range missile launch.

During his stop in Seoul, Kerry had warned North Korea that a launch in the current climate would be a "huge mistake".

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.

But South Korean Defence Ministry spokesman Kim Min-Seok said Monday that the current "missile watch" could drag on for some time.

"We had assumed the North may launch the missile around April 10 but five days have passed already. So we believe the situation may drag on for quite a while," Kim said.

"We are still working on the belief that the North, once a political decision is made, can fire the missile very quickly," he added.