(CNN) -- South Korean President Lee Myung-bak has vowed "resolute" measures against North Korea for its alleged attack on a South Korean warship, South Korea's Yonhap News Agency reported Thursday.
North Korea denied Thursday that it sank the warship in March.
A joint investigation committee announced Thursday morning in Seoul that they had concluded a North Korean submarine fired a torpedo that sank the South Korea warship in March. The committee comprised of American, Australian, British and Swedish, as well as South Korean, experts.
"(We) will take resolute countermeasures against North Korea and make it admit its wrongdoings through strong international cooperation and return to the international community as a responsible member," Lee told Australian Prime Minister Kevin Rudd in phone talks, according to Lee's office, Yonhap reported.
The North Korean National Defense Commission said in a statement to official television that its navy did not torpedo the South Korean ship, calling South Korean President Lee "a traitor," Yonhap reported.
Investigators had recovered a propeller from the torpedo that blew the ship, the Cheonan, in half on March 26, Yoon Duk-yong, the committee's co-chair, told reporters.
The propeller was from the kind of "torpedoes that were exported from North Korea and the letters and the fonts on the torpedo are the are the same that are used by North Korea," Yoon said at a Thursday morning press conference. ".... This torpedo was manufactured in North Korea."
The investigation found that a small- to mid-seized North Korea sub "fired the torpedo that sunk the Cheonan vessel and retreated back to their border," Yoon said.
As the 1,200-ton vessel went down, 46 sailors were lost near disputed waters in the Yellow Sea.
"We had already warned the south Korean group of traitors not to make reckless remarks concerning the sinking of warship Cheonan of the puppet navy," North Korea's National Defence Commission said in a statement, according to the Korean Central News Agency.
"Nevertheless, the group of traitors had far-fetchedly tried to link the case with us without offering any material evidence."
"It finally announced the results of the joint investigation based on a sheer fabrication," the defense commission said, according to the state-run KCNA. The commission called the new report part of a "smear campaign."
The White House backed the report issued Thursday in Seoul, saying it "points overwhelmingly to the conclusion that North Korea was responsible for the attack."
"This act of aggression is one more instance of North Korea's unacceptable behavior and defiance of international law," said a statement by White House Press Secretary Robert Gibbs. "This attack constitutes a challenge to international peace and security and is a violation of the Armistice Agreement."
The statement noted President Barack Obama spoke with South Korean President Lee Myung-bak on Monday and "made clear that the United States fully supports the Republic of Korea, both in the effort to secure justice for the 46 service members killed in this attack and in its defense against further acts of aggression."
"North Korea must understand that belligerence towards its neighbors and defiance of the international community are signs of weakness, not strength," Gibbs' statement said. "Such unacceptable behavior only deepens North Korea's isolation. It reinforces the resolve of its neighbors to intensify their cooperation to safeguard peace and stability in the region against all provocations."