China kept the world guessing Tuesday on whether it would back new sanctions on Iran over a disputed nuclear program as Western powers banked on Beijing to step up pressure on the Islamic republic.

As leaders of 47 nations wrapped up a nuclear security summit in Washington, German Chancellor Angela Merkel said she was hopeful China would throw its support behind a fourth round of UN sanctions against a defiant Tehran.

Her optimism came as the United States reportedly pledged to keep China's oil imports flowing in case Iran retaliated by cutting off energy supplies to the Asian economic powerhouse.

"I see a positive development, even if it is moving slowly and we can't say whether it will lead to sanctions," Merkel told reporters, adding "I'm very hopeful."

"China is now part of the process, even though we can't say clearly what the outcome will be," she said on the sidelines of the landmark atomic summit which agreed to secure loose nuclear materials around the world within four years.

Germany is among the six world powers leading negotiations with Iran to try to get Iranian leaders to rein in their suspect nuclear program, and Merkel again urged Tehran to meet its international commitments.

China has been the most reluctant of the group known as the P5+1 to impose a fourth set of UN sanctions on the Islamic republic, one of China's key trading partners.

There have been conflicting signals about China's stance over tougher sanctions against Iran, which the United States and its allies accuse of covertly working on a nuclear weapon.

Iran says it is pursuing only civilian power.

Chinese President Hu Jintao told the summit that Beijing firmly opposed the spread of nuclear arms, while backing civilian atomic energy.

"We firmly oppose nuclear weapons proliferation and strongly support efforts to enhance international nuclear security," he said.

In unscheduled talks aimed at building a consensus, Obama also met with sanctions skeptics Brazilian President Luiz Inacio Lula da Silva and Turkish Prime Minister Recep Tayyip Erdogan.

Brazil and Turkey, both of whom are reluctant to back sanctions, are non-permanent members of the UN Security Council, whose vote would count in any vote on the issue.

On Monday, a top White House official said Obama and Hu agreed during talks to jointly push for new nuclear sanctions on Iran.

"They are prepared to work with us," said Jeff Bader, Obama's top official responsible for East Asia on the National Security Council, referring to China.

"The two presidents agreed the two delegations should work together on sanctions."

However China, a veto-wielding member of the Security Council, undercut hopes for a consensus when it said sanctions were not a solution.

"China always believes that dialogue and negotiation are the best way out for the issue. Pressure and sanctions cannot fundamentally solve it," foreign ministry spokeswoman Jiang Yu said.

Jiang said China backs a "dual-track strategy" consisting of continued dialogue with Tehran, but at the same time maintaining the possibility of sanctions if talks fail to halt sensitive nuclear work.

Amid the mixed signals, the New York Times reported Tuesday that Obama had vowed to help keep fuel flowing to China if Iran cuts off supplies in retaliation for joining a drive for the UN sanctions.

Obama assured Hu that he was "sensitive to China's energy needs," the Times said, adding that the US administration had already sounded out other oil producers to help reassure Beijing that there would be no drop in supply.

White House advisor on Iran Dennis Ross travelled to Saudia Arabia last year to seek a pledge that it would step in to help China if needed, the paper said.

It also reported that the US president had laid out details of a proposed Security Council sanctions package to Hu -- the fourth that would be imposed on Iran over its suspect enrichment program.

The measures would include denying Iran access to international credit, choking off foreign investment in its energy sector and slapping restrictions on companies owned by the Islamic Revolutionary Guards, the paper said.

World credibility is at stake in the push to impose tougher sanctions on Iran, said US State Department spokesman Philip Crowley.

UN Security Council members had a "special responsibility," he said. "They have to make their own judgement, but we think at this point the credibility of the international community is at stake," he said.

Iranian President Mahmoud Ahmadinejad meanwhile said in Tehran that he was currently drafting a letter to Obama which will be "published in due time."