US President Barack Obama has said that North Korea must be "held to account" for its alleged sinking of a South Korean warship, a tough statement of support for an ally and made at its leader's side.
Mr Obama also said his administration will launch talks with South Korea aimed at resolving remaining issues blocking the completion of a free trade agreement.
After talks with South Korean President Lee Myung-Bak on the sidelines of global summits in Toronto, the US president used some of his strongest language yet about the March sinking of the South Korean warship Cheonan.
An independent report has found that the ship's sinking was caused by a North Korean torpedo.
"There has to be consequences for such irresponsible behaviour," Mr Obama said.
South Korea has already referred the ship sinking to the UN Security Council, which could adopt a resolution condemning the North for the sinking or issue a less stringent presidential statement. Mr Lee said he wants a "strongly worded" resolution.
"We will do all that we can to deter any acts of North Korean aggression levelled against us," said the South Korean leader.
Mr Obama added: "We stand foursquare behind him."