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Luis Moreno Ocampo told reporters that "no state requested our intervention".
Moreno Ocampo's office announced on Monday that he had opened a preliminary investigation into the November 23 shelling of South Korea's Yeonpyeong Island and the sinking of a South Korean warship in March, but it was unclear where the complaints came from.
"We received no official communication," Moreno Ocampo stressed to reporters on Tuesday. "Korean citizens sent to us communications. Students sent to us communications."
He said his office will now conduct a preliminary assessment to determine whether a full-scale investigation of possible war crimes by North Korea should be carried out.
"We have a duty to assess if the court should intervene or not," Moreno Ocampo said.
He said prosecutors must determine whether the incidents constitute war crimes, whether the court has jurisdiction, and whether the South Korean government is taking legal action.
The International Criminal Court, which began operating in 2002, is the world's first permanent war crimes tribunal. Under the treaty, the court can step in only when countries are unwilling or unable to dispense justice themselves for genocide, crimes against humanity or war crimes.
South Korea is one of 114 countries that have ratified the Rome Treaty that established the court, but North Korea doesn't recognize its authority.
Moreno Ocampo said the goal of the preliminary examination is "just to collect information, to understand what's happened."
"I have to understand the alleged crimes. I have to check whether according to the law, are these war crimes or not. I have to understand if it's happening on the territory of (South) Korea. I have to understand if (South) Korea is conducting national proceedings, and after that I have to make a decision," he said.
How long will that take?
"The time is when we are sure - when we are sure that we have to dismiss the situation or to open" an investigation, Moreno Ocampo said. "We have to be sure what to do. We cannot do mistakes."
He said his office will offer all parties an opportunity to send information about the incidents "if they want".
The shelling of Yeonpyeong Island killed two South Korean marines and two civilians. Forty-six South Koreans died in the sinking of the warship, the Cheonan. Moreno Ocampo's statement Monday said it was "hit by a torpedo allegedly fired from a North Korean submarine."