SEOUL (Reuters) - South Korean President Lee Myung-bak called for national solidarity against military aggression by the North on Monday, saying Pyongyang looks for division in the South as an opportunity to strike. Lee has sharpened his language against the North after coming under pressure because of a perceived weak response to two North Korean attacks this year that raised tension on the peninsula to the highest since the 1950-53 Korean War. The clashes have led some analysts to say the chance of a wider conflict is greater than ever. In the latest rhetorical sparring, the North last week threatened a nuclear sacred war and Lee vowed a merciless counterattack against any fresh North Korean attacks as rare large-scale military drills in the South kept tension high. We cant afford to have division of 'you against me in the face of national security, because whats at stake is our very lives and the survival of this nation, Lee said in a national radio address. Lee said it was divided public opinion in the wake of the Norths attack on a South Korean navy ship in March, in which 46 sailors were killed, that had prompted the North to bombard a South Korean island near a disputed sea border last month. North Korea denies responsibility for the ship attack. It is when we show solidarity as one that the North dares not challenge us. Their will to challenge breaks, he said. Lee took office in 2008 and ended a decade of free-flowing economic aid to the impoverished North from his two liberal predecessors, despite acts of violence against the South. But he has come under criticism for being indecisive and ineffective against the Norths military acts. TRIGGER-HAPPY Analysts said the frequency and severity of aggressive North Korean action had pushed Lee against a political wall and he had little choice but to order harsh retaliation which could spark wider conflict. A survey by pollster Realmeter showed support for Lee edged higher, above 45 percent, in a poll conducted after he pressed ahead with a live-fire drill this month despite diplomatic pressure from China and Russia to suspend it. South Koreas Defence Ministry said there would be more defensive and routine drills at sea off all coasts this week, with some live-firing expected, prompting the North to say the peninsula was being pushed to the brink of war. North Koreas main newspaper, Rodong Sinmun, on Monday accused South Korea and the United States of trying to start a war and praised the Norths self-restraint. The US and South Korean trigger-happy elements should not misjudge the will of (the North) to preserve peace but immediately stop their reckless military provocations aimed at screwing up the tension on the Korean peninsula, it said. Government researchers in Seoul say the North could attack five disputed islands under South Korean control off the west coast next year, perhaps in connection with North Koreas succession. Competition in the military to express loyalty for successor Kim Jong-un will intensify, increasing instability and the possibility of sudden provocation against the South, the Institute for National Security Strategy said. Kim is the youngest son of ailing North Korean leader Kim Jong-il.