According to US officials, Hu Jintao, the Chinese president, agreed that the US and China would work together on a draft UN resolution for tougher sanctions against Tehran over its nuclear programme.
The announcement followed 90 minutes of bilateral talks between Hu and Obama on Monday, and could mark something of a diplomatic breakthrough after several months of strained China-US ties.
"The Chinese very clearly share our concern about the Iranian nuclear programme," Jeff Bader, Obama's senior director for Asia on the National Security Council, told reporters.
"They're prepared to work with us," he said, interpreting that willingness as "another sign of international unity" on the Iranian nuclear issue.
However, officials in Hu's delegation were less specific about any agreement, saying only that the US and China shared the "same overall goal" on Iran, while stressing the need for more dialogue.
Foreign Policy magazine's Josh Rogin tells Al Jazeera what the meeting could yield
Iran itself was not invited to the meeting and on Monday the Iranian president lashed out at the summitlabelling it "humiliating to humanity".
"World summits being organised these days are intended to humiliate human beings," Iran's Irna news agency quoted Mahmoud Ahmadinejad as saying.
"These foolish people who are in charge are like stupid, retarded people who brandish their swords whenever they face shortcomings, without realising that the time for this type of thing is over."
Iran insists its nuclear programme is for peaceful purposes, and Ahmadinejad has called his own alternative nuclear summit in Tehran on Saturday.
Obama has framed the two-day Washington summit as a key part of his broader agenda for ridding the world of nuclear weapons, and preventing weapons-grade material from falling into the hands of terrorists.
|Ahmadinejad has called an alternative nuclear summit in Tehran on Saturday [AFP]
At the start of the meeting US officials welcomed what they labelled a "landmark decision" from the Ukrainian government to surrender its stockpile of highly-enriched uranium, one of the key elements in making nuclear weapons.
Al Jazeera's Patty Culhane, reporting from the Washington summit, said nuclear experts believe there is about 500 tons of plutonium and 1,600 tons of highly-enriched uranium scattered around in the world.
The majority of it is well guarded, but only a few kilos is required to make a working nuclear bomb US officials believe they have strong evidence pointing to efforts by al-Qaeda and others groups trying to get hold of it.
"Al-Qaeda has been engaged in the effort to acquire a nuclear weapon for over 15 years, and its interest remains strong today," John Brennan, a senior counterterrorism official in the Obama administration, told Al Jazeera.
US officials hope leaders participating in the summit will agree on a series of security steps for their own nuclear material, and help pay to put the stocks of less well-off countries under lock and key.
On Sunday, Obama told world leaders arriving in Washington that the possibility of a terrorist organisation obtaining a nuclear weapon was "the single biggest threat" to global security.
|Obama said groups like al-Qaeda would have no compunction in using nuclear bombs [AFP]
"We know that organisations like al-Qaeda are in the process of trying to secure nuclear weapons or other weapons of mass destruction, and would have no compunction at using them."
The Ukrainian decision to give up its uranium stockpile was announced by Robert Gibbs, the White House press secretary, who said some details of the plan had yet to be worked out, including how and where the uranium will be disposed of.
Gibbs said the material could be sent to the US or Russia.
He declined to specify the amount of uranium involved, other than to say it was enough to make several nuclear weapons.
The announcement followed a private meeting between the US and Ukrainian presidents, during which Obama reportedly praised the agreement as historic and pledged US technical and financial assistance to support it.
Obama has set a goal of securing all vulnerable nuclear materials worldwide within four years – an objective he hopes will be endorsed by all parties attending the Washington summit, even if the means to accomplish it remain unclear.
The summit comes just days after Obama signed a new nuclear arms reduction treaty with Russia, and ahead of next month's review of the Nuclear Non-Proliferation Treaty.