The prosecutor of the International Criminal Court said today his office was starting a preliminary examination of possible war crimes by North Korea.
The United Nations probe follows complaints from South Korean students and citizens.
Luis Moreno Ocampo said that "no state requested our intervention".
Mr 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," Mr Moreno Ocampo said today. "Korean citizens sent to us communications. Students sent to us communications."
He said his office would now conduct an 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," Mr Moreno Ocampo said.
He said prosecutors must determine whether the incidents constituted war crimes, whether the court had jurisdiction, and whether the South Korean government was taking legal action.
The UN's 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 does not recognise its authority.
Mr Moreno Ocampo said the goal of the preliminary examination was "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.
"The time is when we are sure - when we are sure that we have to dismiss the situation or to open" an investigation, he said. "We have to be sure what to do. We cannot do mistakes."
He said his office would 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. Mr Moreno Ocampo's statement on Monday said it was "hit by a torpedo allegedly fired from a North Korean submarine".
North Korea launched apparent artillery drills today as top US and South Korean military leaders held talks on the peninsula's security worries following the deadly North Korean artillery strike last month.
As the US chairman of the Joint Chiefs of Staff Admiral Mike Mullen and his South Korean counterpart General Han Min-koo met behind closed doors in Seoul, North Korea staged what appeared to be firing exercises near the disputed western sea border.
North Korean shells landed in the country's own waters north of South Korea's Baengnyeong island, a South Korean military official said.
North Korea also carried out an apparent military exercise within sight of the South's Yeonpyeong Island last month following the artillery assault on the island. Artillery shots were heard three days later as General Walter Sharp, the top US commander in South Korea, toured the island in a show of solidarity with Seoul and to survey damage.