Syrian rebels meet US Secretary of State John Kerry and G8 foreign ministers in London on Wednesday as the United States mulls ways to step up support for outgunned opponents of President Bashar al-Assad.

The Democratic People's Republic of Korea's threats of war will also be high on the agenda of the Group of Eight leading industrialized nations - the United States, Britain, France, Germany, Italy, Japan, Canada and Russia - meeting in the British capital.

The talks, which formally began over dinner on Wednesday and is scheduled to end on Thursday, will also be the first chance for the ministers to speak face-to-face after last week's meeting in Kazakhstan on Iran's nuclear issue.

Kerry met Syrian opposition leaders and Russia's top diplomat a day after saying the US could soon step up aid to rebels fighting Assad's government.

Kerry attended a lunch alongside the Syrian opposition's interim prime minister and other senior figures. He later met one-on-one with Russian Foreign Minister Sergey Lavrov.

Leaders of the Syrian National Coalition will be present on the sidelines of the G8 and hold talks with those foreign ministers willing to meet them, British Foreign Secretary William Hague told reporters on Tuesday.

The radicalization of elements of the divided opposition has tempered the enthusiasm of some Western nations for supporting the rebels militarily.

Iraq's al-Qaida wing said on Tuesday it had united with Syria's al-Nusra Front, a kindred group.

Michael Stephens, a Doha-based analyst for security think tank RUSI, said the presence of SNC leaders at the G8 meeting was designed to help the coalition shore up its international legitimacy after the Arab League recognized it as the sole representative of Syria.

"It shows there's a graduated process, where they went from laughing stock to being approved by the Arab League to being listened to by G8 leaders," he said. "There's a groundswell of support that appears to be building up behind them."

France and Britain are expected once again to press the case for amending or lifting an arms embargo on Syria to support the outgunned rebels and help end a conflict that has claimed an estimated 70,000 lives and displaced millions of people.

"This is turning into the greatest humanitarian catastrophe of the 21st century so far, and we cannot watch this happen," Hague said, adding that humanitarian aid, while indispensable, would not alone solve the Syrian crisis.

"We certainly believe that it's necessary to continue, if the situation continues to deteriorate, to increase the practical help we give to the Syrian opposition," Hague said.

Paris and London say Assad could be forced to the negotiating table by allowing the supply of arms to the rebels.

Russia and Germany oppose such a move, arguing it could lead to weapons falling into the hands of Islamist militants and fuel a regional conflict.

The foreign ministers also discussed how to respond to the DPRK, which has been threatening to attack the United States and the Republic of Korea, and on Tuesday warned foreigners to evacuate the ROK to avoid being caught in a war.

Lavrov said on Wednesday that Moscow agrees with Washington's stance on DPRK but warned that any military moves could hamper chances to calm tensions in the Korean peninsula, the Interfax news agency reported.

"There is no disagreement with the US over North Korea," Lavrov said at a meeting with Kerry in London, where G8 foreign ministers are to hold talks.

"One should not frighten anybody with military manoeuvres," he was quoted as saying, apparently referring in part to past military exercises by the US and DPRK. "There is a chance everything will calm down."

Hague said the ministers also planned to discuss Myanmar, Somalia, cyber-security and sexual violence in war zones.

Reuters-AP