The onus is on the government in Tehran to disclose more about its nuclear activities, the G8 leaders agreed at Friday's opening dinner, hosted by U.S. President Barack Obama.

The meeting, at Camp David near Washington, comes ahead of a fresh round of international talks with Iran, to be held in Baghdad.

The leaders, including France's newly elected President Francois Hollande and Russian Prime Minister Dmitry Medvedev, also stressed the need to move rapidly toward a plan for political transition in Syria, the senior White House official said.

"Exactly what the nature of the transition is will be worked through in coming days," the official said.

Medvedev, who is filling in for President Vladimir Putin, "did not dispute the need for some type of political process in the country that would be responsive to the Syrian people," he added.

Putin said he could not attend as he was busy finalizing his cabinet.

On Friday, French President Hollande said he would put pressure on Moscow to back new UN Security Council Russia on Syria.

Russia has vetoed two UN Security Council resolutions on Syria over what it says is a pro-rebel bias, but gave its full backing to a UN-backed peace plan in March.

Leaders also said North Korea needed to adhere to international norms for nuclear matters and that it would face deepened isolation if it "continues down the path of provocation"

At the end of the dinner, Obama gave a chocolate birthday cake to Japanese Prime Minister Yoshihiko Noda, whose birthday is on Sunday.