In a display of unshakeable unity against nuclear-armed North Korea, US President Barack Obama agreed Saturday to wrap up a long delayed free trade deal with South Korea and extend Washington's wartime command of South Korean forces.

As tensions surged on the Korean peninsula after Pyongyang was blamed for a deadly attack on a Seoul warship, Obama held warm talks with South Korean President Lee Myung-Bak, declaring that "our friendship and alliance continues to grow" both on the security and economic fronts.

After talks on the sidelines of a G20 summit, Obama launched a new initiative to implement a free trade agreement signed between the two allies in June 2007 during the administration of his predecessor George W. Bush.

Its implementation had been put on hold after Obama raised market access problems over American beef and autos.

On Saturday, Obama ordered his officials to complete talks with their South Korean counterparts to get the agreement ready by November, when he visits Seoul for the next G20 summit.

"I want to make sure that everything is lined up properly by the time I visit Korea in November, and in the few months that follow that, I intend to present it to Congress," Obama said.

"It is the right thing to do for our country, it is the right thing to do for Korea," the US leader said.

The trade deal has been touted as the biggest free trade agreement since the 1993 North American Free Trade Agreement (NAFTA).

In their joint press conference after talks, Obama and Lee also sent a firm message to North Korea that it should be held to account for its alleged torpedo attack on a South Korean ship which left 46 South Korean sailors dead.

"We stand foursquare behind him," Obama said as he met with Lee, who vowed Seoul "will react swiftly and strongly" to prevent another attack by North Korea.

Tensions on the Korean pensinsula rose after a multinational investigation found the North torpedoed the 1,200-tonne Cheonan near the disputed Yellow Sea border.

Pyongyang has angrily denied any responsibility and threatened military retaliation if it is slapped with international censure amid concerns that it may be preparing for a new round of missile tests..

Obama and Lee also agreed Saturday to postpone until 2015 Washington's transfer of wartime command of allied South Korean forces to Seoul.

Currently, if war were to break out on the Korean peninsula, the United States would assume operational command of South Korean forces. Under a 2007 agreement with Seoul, this plan was due to come to an end in April 2012.

The White House's chief adviser on Asia, Jeff Bader, told reporters on a conference call that South Korea wanted to push back the date to underline the US commitment to security in the region at a time of tension.

In another show of support for South Korea, leaders of the Group of Eight powerful nations condemned Saturday the alleged North Korean strike and sought "appropriate measures" against "those responsible for the attack."

"We demand that the Democratic People's Republic of Korea (DPRK) refrain from committing any attacks or threatening hostilities against the Republic of Korea," it added.

The fact that Russian President Dmitry Medvedev endorsed the G8 statement is viewed by some Asian diplomats as a blow to North Korea and China.

China and Russia, veto-wielding members in the Security Council, are closer to the reclusive North Korean regime and have so far refused to cast judgment on the investigation until they assess the findings themselves.

Except for China, all permanent members of the council -- Britain, France, Russia and the United States -- are part of the G8.

South Korea and the United States are pressing the UN Security Council to censure the hardline communist state over the incident.

A US official said the G8 statement "provides very strong momentum which we want to build on in New York."