Leaders of the industrial world turned their sights on Iran, Afghanistan and North Korea on Saturday, as security threats and nuclear proliferation took center stage at the G8 summit.

"The session ... is going to focus on peace and security, Iran and North Korea will be discussed" a senior US official told reporters at the Group of Eight summit being held north of the Canadian city of Toronto.

The official, who spoke on condition of anonymity, said US President Barack Obama planned meetings with the leaders of South Korea on Saturday and of China and Japan on Sunday to discuss the security situation in East Asia.

Summit host Canada urged the leaders of fellow G8 powers Britain, France, Germany, Italy, Japan, Russia and the United States to jointly rebuke North Korea over the sinking of a South Korean warship.

"Canada's view is that there has to be a strong condemnation of these attacks and that ... we must state strongly that they must not carry on such attacks in the future," said Canadian government spokesman Dimitri Soudas.

In March a South Korean warship sailing near the country's disputed maritime boundary with North Korea sank with the loss of 46 sailors.

An international investigation has established that the vessel was sunk by a torpedo fired by a North Korean submarine. The isolated Pyongyang regime denies this and has threatened to respond to any sanctions with military force.

Japan and the United States are also pushing for an outright condemnation of the nuclear-armed authoritarian state.

Four of the five permanent UN Security Council members -- Britain, France, Russia and the United States -- are in the G8.

Following the G8 summit the leaders will be joined with more colleagues from major developed and emerging economies to form the larger G20 and continue their talks into Sunday.

British officials said Canada had asked British Prime Minister David Cameron, attending his first G20, to lead a discussion on the fighting in Afghanistan, where NATO troops are battling Taliban rebels.

"This will be an important opportunity for the prime minister to reaffirm strong support for the current strategy, to talk about the importance of a political strategy to go alongside the military campaign and to call on the G8 to maintain its level of support," Downing Street said.

On Friday, Europe and the United States tried to bridge differences over how to sustain fragile global economic recovery and sought common ground on dealing with ballooning deficits.

There had been fears of a potential clash between European leaders bent on slashing spending and a Washington fearful of stifling growth.

Germany's Chancellor Angela Merkel insisted members must move fast to cut soaring public deficits and ensure financial stability -- but both she and US officials stressed this did not represent a split with the United States.

"I have made it clear that we need sustainable growth and that growth and intelligent austerity measures don't have to be contradictions," Merkel said.

A senior US official, speaking on condition of anonymity, confirmed the meeting had gone well and that Merkel and Obama had not fallen out over Germany's call for immediate fiscal tightening.

"The president sees deficit reduction as part of a medium and long-term growth strategy. Coming to the G8 and G20 his main focus is these things are not exclusive," the administration official told reporters.

In Toronto around 2,000 protesters -- a loose coalition of leftist activists and anarchists -- faced off against riot police on Friday, but there was no serious violence and no more than a handful of arrests.

Larger protests were planned for Saturday, when delegates return to Toronto from the Huntsville resort to meet more world leaders under the G20 format.

Canada has spent a billion dollars to secure the summit behind a ring of steel and police reinforcements, hoping to avoid a repeat of the large-scale street violence that has marred previous global meetings.