US President Barack Obama on Tuesday said Russia and China understood the dangers of a Syrian civil war but had not yet "signed on" to a plan to prod President Bashar al-Assad from power.

Obama said after meeting Presidents Vladimir Putin and Hu Jintao at the G20 summit that he hoped work could take place on a political transition plan to present to the Syrian people in the next two weeks.

But he admitted Beijing and Moscow still were not on board with the Western position that Assad must go, and Putin bluntly declared: "Nobody has the right to decide for other nations who should be in power and who should not."

Obama said he had candid and intense discussions with Putin and Hu on Syria at the summit in Mexico, and took pains in a press conference to show that he understood their positions and reservations.

"I wouldn't suggest that at this point the United States and the rest of the international community are aligned with Russia and China... but I do think they recognize the grave dangers of all out civil war," Obama said.

The US leader made clear that he had told Putin and Hu on the sidelines of the G20 summit that Assad could no longer remain in power after massacring large numbers of civilians.

He admitted that Russia's historical ties with Damascus and China's reluctance to comment on internal affairs of other countries represented an impediment to solving the Syrian crisis.

"I don't think it would be fair to say the Russians and Chinese are signed on at this point... they recognize the current situation is grave... it does not serve their interests," Obama said.

"I do not think they condone the massacres that we have witnessed and I think they believe everybody would be better served if Syria had a mechanism for ceasing the violence and creating a legitimate government."

Such comments represented a significant political risk for Obama.

His critics in the United States, including presidential foe Mitt Romney, have hammered him for "resetting" relations with Russia while Putin stands accused of facilitating the carnage perpetrated by Assad against his people.

Obama said it was now important for world powers to work with UN envoy Kofi Annan to in the "coming week or two" frame a plan to present to "the Syrian people a pathway whereby this conflict can be resolved."

However, even if sharp disagreements between Washington and Russia and China could be resolved on Syria, it remains highly uncertain that a unified plan sketched by the outside world could dislodge Assad.

Obama said he made clear to Putin and Hu that the Syrian president's days are numbered: "I don't see a scenario in which Assad stays and violence is reduced."

France's President Francois Hollande, who was also in the Mexican resort of Los Cabos for the summit, said that he thought Putin was ready to "play his role" in the political transition in Syria.

But this was contradicted by the Kremlin leader, who said: "It is not changing the regime that is important, but that after changing the regime, which should be done constitutionally, violence is stopped and peace comes to the country."

Putin said all sides should sit down and work things out beforehand.

And, in a veiled reference to simmering unrest in Libya in the wake of the NATO-backed ouster of now-slain dictator Moamer Kadhafi, added: "Unlike in some North African countries where violence goes on even after regime change."