(CNN) -- North Korea accused the South Korean navy of trespassing into its waters Tuesday and threatened to retaliate with military action, the Yonhap news agency said.
Tensions are high between the two neighbors since an official South Korean report which blamed the Communist North of firing a torpedo at a navy ship, killing 46 sailors.
Yonhap news agency did not release any other details on the alleged threat.
Secretary of State Hillary Clinton urged North Korea Monday to reveal what it knows about the "act of aggression" that sunk a South Korean warship.
She also said the United States' "support for South Korea's defense is unequivocal" and that North Korea should "stop its belligerence and threatening behavior."
South Korea has said a probe concluded the North fired a torpedo that sunk a South Korean military ship in March. The United States supports that finding, Clinton said while in China.
South Korean President Lee Myung-bak announced Monday that his country was suspending trade with North Korea, closing its waters to the North's ships and adopting a newly-aggressive military posture toward its neighbor.
"We endorse President Lee's call on North Korea to come forward with the facts regarding this act of aggression and, above all, stop its belligerence and threatening behavior," Clinton said.
President Barack Obama has directed military commanders to work with South Korean troops "to ensure readiness and to deter future aggression" from North Korea.