Authorities in Seoul inspecting the debris on Yeonpyaong Island after the North Korea attack suspect that thermobaric weapons were used. The bombs are aimed at increasing the number of casualties and causing severe structural damage at the same time. Commonly known as the fuel-air bombs, they have been in usage since 1985 and are capable of sending a blast wave for significantly longer duration than those produced by condensed explosives.

The Yonhap news agency quoted a military source as saying, After analyzing debris of artillery rounds on Yeonpyeong Island, we preliminarily concluded that some of them were a sort of special weapon similar to thermobaric bombs.

The official added that the North used such weapons to kill a number of people and maximize confusion by causing fire.

Two soldiers and two civilians were killed while several others were injured as North Korea fired artillery shells at a South Korean island Yeonpyaong Island in the Yellow Sea on Tuesday. The Island located near the West Sea inter-Korean border has often been a place of military hostility between the both the nations. Pyongyang and Seoul have blamed each other for setting off the artillery attacks.

In view of the tensions in the region, South Korea stated that it will sharply increase the military force in the five northernmost isles of the Yellow Sea. The Presidential office said on Thursday that the Armed forces of the country are set to revise the rules of engagement. A security review was held by President Lee Myung-bak in Seoul and the government decided to equip the islands with modern weaponry.

There was an assessment that the existing rules of engagement were rather passive, as it was mainly concerned with preventing escalation of the clash, Hong Sang Pyo, the presidential spokeswoman said. This kind of provocation can recur anytime, she added.

The country's military has been under fire as Opposition blamed it for a slow and inadequate response to the North's Tuesday attack.

Meanwhile, North Korea threatened retaliatory attacks on South Korea if it resorted to further military provocations.

According to the North's official KCNA news agency, a statement sent to the U.S. forces stationed in South Korea said, Korean People's Army will deal without hesitation the second and third strong physical retaliatory blow.

The statement also accused the United States of sparking off the conflict in the region and maintained that the South Korean forces violated the Armistice Agreement.