The US and Russian presidents have acknowledged at the G8 meeting in Northern Ireland they have a widely different stance on Syria, but did agree to push for a summit in Geneva.
After face-to-face talks, Barack Obama and Vladimir Putin said they shared a common desire to end the violence.
Both also said they were optimistic on Iran, after its presidential election.
Earlier, the G8 nations discussed the global economy, with the leaders agreeing world prospects remained weak.
The G8 leaders are now heading to a working dinner, where Syria is likely to be top of the agenda.
Other nations joining the UK, US and Russia for the 39th Summit of the Group of Eight (G8) in Lough Erne, County Fermanagh, are Canada, France, Germany, Italy and Japan.
Mr Putin and Mr Obama met for about two hours on the sidelines of the summit.
Correspondents say that both leaders looked tense as they addressed journalists afterwards, with the Russian president regularly looking at the floor.
Mr Putin said: “Our positions do not fully coincide, but we are united by the common intention to end the violence, to stop the number of victims increasing in Syria, to resolve the problems by peaceful means, including the Geneva talks.”
Mr Obama said the two leaders had instructed their teams to press ahead with trying to organise the peace conference in Switzerland.
Neither the rebels nor the Syrian government have yet fully committed to the proposed Geneva talks, which would seek to end more than two years of unrest that has left an estimated 93,000 people dead.
Mr Obama and Mr Putin did say that they had agreed to meet in Moscow in September.
Earlier UK Prime Minister David Cameron, who is hosting the summit, had said he hoped to find “common ground” on Syria.
The US said last week it was prepared to arm opposition forces, saying it had evidence that President Bashar al-Assad’s forces had used chemical weapons on a “small scale”.
Mr Cameron, who backed the recent lifting of EU arms sanctions against the rebels, said on Monday that no decision had yet been made on whether the UK would do the same.
In an interview in Germany’s Frankfurter Allgemeine Zeitung on Monday, Mr Assad denied that his military had used chemical weapons, and warned that arming the rebels would result in “the direct export of terrorism to Europe”.
“Terrorists will return to fight, equipped with extremist ideology,” he said.
On Monday, Russian foreign ministry spokesman Alexander Lukashevich said of the possibility of a no-fly zone over Syria: “I think we fundamentally would not allow this scenario.”
The formal talks on Monday covered the global economy.
In their statement after the session, the leaders said prospects remained weak but added that action in the US, Japan and eurozone had helped ease the situation.
“Downside risks in the euro area have abated over the past year, but it remains in recession.
“The US recovery is continuing and the deficit is declining rapidly in the context of a continuing need for further progress towards balanced medium-term fiscal sustainability.”
Ahead of the first session, the US and EU members of the G8 announced that negotiations were to begin on a wide-ranging free-trade deal.
Mr Cameron, Mr Obama and European Commission President Jose Manuel Barroso held a press conference on the proposed EU-US deal.
Mr Cameron said a successful agreement would have a greater impact than all other world trade deals put together.
“This is a once-in-a-generation prize and we are determined to seize it,” said Mr Cameron.
He said the deal “could add as much as £100bn ($157bn; 117bn euros) to the EU economy, £80bn to the US economy and as much as £85bn to the rest of the world”.
Mr Obama said the deal was a priority for the US and he hoped that it would create an economic alliance as strong as the diplomatic and security alliances the two sides enjoyed.
President Obama, on his first visit to Northern Ireland, delivered a public address at the Waterfront Hall centre in Belfast before travelling on to Lough Erne.
On Tuesday, Mr Cameron will hope to make progress on tax transparency after agreeing a deal on the issue over the weekend with British overseas territories and Crown dependencies.
He has made no secret of his desire to tackle tax evasion and aggressive tax avoidance.
Tuesday will also cover counter-terrorism issues.
As the event started allegations were made in The Guardian newspaper on Monday, that Britain had spied on delegates who attended two G20 meetings in London in 2009.
The newspaper reported that documents, leaked by the ex-CIA whistleblower Edward Snowden, showed that British intelligence monitored the computers of foreign politicians and officials.
British spies are accused of setting up internet cafes to read delegates’ email traffic and penetrating the security on officials’ BlackBerrys to monitor email messages and phone calls. Targets are alleged to have included the Turkish finance minister and possibly 15 others in his party.
The Turkish foreign ministry said the allegations, if true, would “constitute a scandal”.
Observers say the revelations could cause tensions among delegates attending the G8, but Mr Cameron refused to be drawn, saying he “never comments on security and intelligence issues”.
Some 8,000 police officers are being deployed for the summit.
On Monday, up to 1,000 protesters began a march from Enniskillen to the summit site, championing a range of causes from anti-globalisation to gay rights.
The colourful parade set off it festive spirit and organisers said they did not expect any trouble.