An undated photo released in January 2009 shows a missile firing drill at an undisclosed location in North Korea.
May 30: South Korean marines man at their positions at the South Korea's western Yeonpyong Island, near the disputed sea border with North Korea.
May 29: South Korean Navy sailors on speed boats aim their machine guns at Yeonpyong Island, west of mainland South Korea.
May 26: North Koreans are seen during a ceremony to celebrate the nation's underground nuclear test, in Pyongyang, North Korea.
May 24: A map of North Korea shows the epicenter of a 4.7 magnitude earthquake believed to have occurred as a result of the nuclear test.
April 9: North Korean leader Kim Jong II attends the first session of Supreme People's Assembly of the country.
South Korea's president warned Saturday that his country will not let North Korea use its recent nuclear and missile tests to win concessions and that Pyongyang needs to return to talks aimed at ending its nuclear program.
The communist nation has escalated tensions in the region in recent weeks after conducting its second nuclear test and launching a barrage of missiles. The North also appeared to be preparing to test a long-range ballistic missile capable of reaching the U.S.
"I would like to make it clear that there won't be any compromise" with North Korea when the country threatens the South's security, President Lee Myung-bak said in a speech marking Memorial Day at a national cemetery in Seoul.
Lee did not elaborate on what kind of compromises he was referring to. But some experts believe the North is using its nuclear and missile tests, and the trial of two American journalists, to strengthen its position for possible talks with the U.S. in order to win concessions or much-needed economic aid.
"North Korea should keep its promise to denuclearize the peninsula and come forward to inter-Korean dialogue," Lee added.
Lee also called on the military to have perfect defense readiness during his tour to Osan Air Base near Seoul, where he met top South Korean and U.S. military officials, according to his office. The U.S. has some 28,500 troops in South Korea to help deter the possible North's aggression.
His warning came as seven key nations sent a draft U.N. resolution with proposed new sanctions against North Korea to their governments for review. Ambassadors were expected to discuss reactions from their governments next week.
Closed-door meetings have been held since May 26, a day after North Korea's underground atomic blast.
The U.S. will impose its own financial sanctions on the North apart from possible U.N. punishments, South Korea's Chosun Ilbo newspaper reported Friday. The U.S. moves call for blacklisting foreign financial institutions that help the North launder money and conduct other dubious deals, it said.
U.S. Deputy Secretary of State James Steinberg briefed Lee on the sanctions at a meeting Thursday, the paper said, citing an unidentified official at the presidential office. The U.S. Embassy in Seoul could not confirm the report.
Many analysts believe economic sanctions against the isolated North will not be effective unless China actively implements them. Pyongyang relies heavily on China for food and energy aid and imports. More than 70 percent of the North's total trade is with China.
A U.S. measure imposed in 2005 on Banco Delta Asia in the Chinese territory of Macau effectively led to the North being severed from the international financial system, as other institutions voluntarily severed their dealings with the bank and the North. Pyongyang stayed away from nuclear disarmament talks for more than a year in retaliation.
Complicating matters, two U.S. journalists were believed to be standing trial in North Korea, but Pyongyang has released no information since the scheduled start of the proceedings Thursday. They were arrested near the China-North Korean border three months ago and accused of "hostile acts" and illegally entering the country.
In Washington on Friday, Secretary of State Hillary Clinton called on the North to release the women and allow them to return home.
Separately, Lee also renewed calls on North Korea to release a South Korean man being held in the North over allegations he slandered the communist regime. He was detained in late March at a joint industrial complex in the northern border town of Kaesong but North Korea has denied Seoul's access to him and his whereabouts remain unclear.