US President Barack Obama on Tuesday lobbied President Hu Jintao of China, a veto-wielding permanent member of the United Nations Security Council, on efforts to end the bloodshed in Syria.

China and Russia have frustrated the United States by blocking stronger UN sanctions and action against President Bashar al-Assad, and Obama has partly used the G20 summit in Mexico to push for a change in attitude.

Obama told reporters at the start of the talks in the Mexican resort of Los Cabos that he wanted to discuss with Hu ways "that can end the bloodshed and arrive at the kind of legitimate government that I think we all hope for."

On Monday, the US leader had spent a third of a two-hour meeting with Russian President Vladimir Putin dealing with Syria, and the leaders agreed a political transition is needed in the country but failed to come up with an immediate plan.

Both Russia and China said earlier this month that they would coordinate in backing the peace plan for Syria framed by UN envoy Kofi Annan, despite its failure to halt a brutal crackdown on opposition forces and civilians.

The two nations supported the UN Security Council move to strongly condemn the Syrian government for using artillery in a massacre in the central town of Houla in which at least 108 people were killed.

But they have vetoed two UN Security Council resolutions highly critical of Assad's regime and they oppose any military intervention in Syria or efforts to enforce regime change.

Obama, who last met Hu on the sidelines of a nuclear security summit in Seoul in March, said he also wanted to discuss "Iran, North Korea and the challenges of curbing nuclear proliferation."

Hu, speaking through a translator, mentioned that Tuesday's encounter was the 12th between the two leaders and said the two sides had made "made new progress in the cooperative partnership with the United States."

Obama's talks with Hu were politically sensitive, given that they took place less than five months before the US election, as his Republican foe Mitt Romney accuses him of not standing up to Beijing on economic clashes.

Critics in the United States accuse China of keeping its currency deliberately undervalued against the dollar to boost its exports and of infringing the rules of global trade in a way that hurts US workers.