Toronto - President Barack Obama said Saturday 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.

Obama also said Saturday 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 here, the U.S. 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," Obama said.

South Korea has already referred the ship sinking to the U.N. Security Council, which could adopt a resolution condemning the North for the sinking or issue a less stringent presidential statement. 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.

Said Obama : "We stand foursquare behind him."

Also pleasing Lee, Obama announced that he would submit to Congress early next year a long-upheld free trade agreement with South Korea.

He said the U.S. will resume talks with South Korea to resolve issues that have blocked the accord. Those discussions will take place between now and the Group of 20 international summit, to be held in Seoul in November. Obama said he would present the accord to Congress within a few months after that.

"It is the right thing to do for our country. It is the right thing to do for Korea," Obama said, with Lee at his side.

Obama said a completed deal would strengthen commercial ties between the two countries and "create enormous potential economic benefits" by removing barriers to the sale of American products in South Korea, the 14th largest economy in the world.

Obama said the agreement would "create jobs here in the United States which is my No. 1 priority."

The administration of George W. Bush negotiated a free trade deal with South Korea in 2007, but the agreement has been stalled in the United States since that time because of determined opposition led by U.S. automakers who contend that the agreement did not go far enough to remove barriers to the sale of U.S. cars in Korea.

Since taking office in January 2009, the Obama administration, in the face of stiff opposition from U.S. labour groups, has refused to send the agreement to Congress for a vote.

The tough talk toward North Korea by Obama and Lee was sure to be a topic in the U.S. president's next meeting, with Chinese President Hu Jintao.

Since China is a veto-wielding member of the Security Council, its support is required for any resolution. But Beijing, as North Korea's main ally, has resisted punishment for Pyongyang.

However, neither Obama nor Hu described any specifics they had in mind for their discussions , sticking to platitudes about often-difficult relations between the two powers when they briefly spoke to reporters before their meeting.

"It's my belief that we've accomplished many things," Obama said.

"Real progress has been made in this relationship," Hu said.

Obama also said that he and Lee had agreed, at South Korea's request and in light of the ship sinking, to delay handing off wartime operational control of Korean forces on the Korean Peninsula from the U.S. to Korea until the second half of 2015. This gives "appropriate time ... to do this right," Obama said.

Korea was to have assumed control of its own forces in 2012 in the event of war.

Obama began his day by concluding a summit of the leading eight industrial democracies at a resort in Canada's forested Muskoka lakes region, then flew here for a second, expanded summit of the so-called Group of 20 nations. Those larger meetings, mostly being held on Sunday, were getting underway with a dinner. In between, Obama was meeting on the sidelines with important foreign counterparts.