SEOUL—South Korea on Thursday will formally blame North Korea for sinking a South Korean warship and killing 46 of its sailors, and the South's foreign minister said Wednesday it had marshaled enough evidence to seek international penalties.
North Korea has been suspected from the moment an explosion tore apart the Cheonan patrol ship, sinking it near the inter-Korean border in the Yellow Sea on March 26.
But South Korean officials waited until recovering the ship's bow and stern to explicitly state that a torpedo caused it to explode. And now, after three weeks of examination by investigators from the U.S., Britain, Australia, Sweden and South Korea, the Seoul government is ready to officially point the finger at Pyongyang.
The Cheonan's stern was raised to probe the cause of its March sinking.
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South Korean defense officials have described some of the findings in recent weeks, including how satellite photography of a North Korean port showed a submarine was missing for several hours before and after the Cheonan sank. Investigators also found explosive residue and metal fragments of a torpedo that matched one of North Korean origin, officials have said.
Several South Korean newspapers on Wednesday reported that investigators found a torpedo propeller with lettering that appears to be from North Korea. A spokesman for the defense ministry declined to confirm those reports, however.
Also on Wednesday, Foreign Minister Yu Myung-hwan told reporters it was "obvious" that North Korea sank the patrol boat. "We will take appropriate measures in a firm and cautious manner," Mr. Yu said.
He added that the South's investigation yielded enough details to seek a penalty at the United Nations Security Council but that it was premature to discuss such action.
The North has blamed South Korea for the incident but hasn't explicitly denied involvement.
Senior government officials, including Mr. Yu and Defense Minister Kim Tae-young, on Tuesday listened to investigators describe what they found. President Lee Myung-bak, who received a separate briefing, plans to speak on the matter in a national address next week, a spokesman said.
South Korean officials are expected take several more weeks to seek penalties, starting with some form of censure or sanctions imposed by the U.N. North Korea reacted harshly to U.N. sanctions imposed after it tested a nuclear weapon last May. Some analysts say North Korea may undertake another nuclear test in the face of further international condemnation.
Seoul is set to accuse North Korea of killing 46 sailors, mourned above in a late April service, in a torpedo attack.
South Korean officials have also been discussing unilateral penalties, including cutting the few remaining economic connections with North Korea, restarting official government-sponsored radio broadcasting aimed at the North and changing some of the procedures its military uses when confronting the other country.
But neither South Korea nor the U.S. have much ability to punish North Korea militarily because of the North's long-held ability to attack the South with hundreds of rockets and missiles. South Korea's capital, Seoul, is just 30 miles from the inter-Korean border.
Senior U.S. officials said Secretary of State Hillary Clinton, who is scheduled to leave Thursday for a visit to the region, will raise the issue while in Beijing, Tokyo and Seoul.
Washington wants to demonstrate a strong response to Pyongyang without risking a military escalation between the two Koreas, the U.S. officials said. They said one casualty of the Cheonan issue will likely be the six-party negotiating process over North Korea's nuclear program, which includes the U.S., China, South Korea, North Korea, Japan and Russia. Pyongyang has refused to take part in the talks for over a year, instead seeking international recognition as a nuclear-weapons state.
Mrs. Clinton will also face the delicate task of discussing the Cheonan matter with officials in China—North Korea's largest trade partner, economic benefactor and supporter. Seoul was irked when Beijing waited nearly a month to offer condolences for the South Korean sailors killed. Chinese President Hu Jintao two weeks ago hosted North Korean leader Kim Jong Il in Beijing.
North Korea's state media in recent weeks issued critical statements in response to South Korean news reports about the Cheonan investigation.
Pyongyang said Tuesday that its parliament would convene early next month for the second time this year, an unusual step for a group that usually meets once a year to rubber-stamp actions taken by Mr. Kim. North Korea didn't say why the group would meet, though analysts in Seoul speculated on a connection to findings in the Cheonan matter.
Tensions between the Koreas have remained high since the Cheonan's sinking. On Sunday, a North Korean fishing boat and naval patrol boat crossed south of the inter-Korean maritime border. The fishing boat turned back after South Korea's military issued a radio warning. The military vessel returned only after a South Korean patrol boat fired two warning shots.
—Jaeyeon Woo in Seoul and Jay Solomon in Washington contributed to this article.