Russia's president Vladimir Putin and his US counterpart Barack Obama have agreed to look for a diplomatic end to the Syrian civil war but clashed over the central question of whether Syrian leader Bashar al-Assad should retain power.

During a 90-minute meeting, Mr Putin and Mr Obama agreed their armed forces should hold talks to avoid coming into conflict in Syria after a Russian military buildup there over the past several weeks.

The US, France and allied countries are bombing Islamic State militants, who have exploited power vacuums to seize parts of Syria and neighboring Iraq as part of a stated goal of creating an Islamic caliphate.

The reinforcement of the Russian military presence in the country, including the addition of tanks and warplanes, has brought fears of inadvertent or accidental clashes among the forces as well as US questions about Moscow's main goal.

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Speaking after his meeting with Mr Obama, Mr Putin told reporters Russia was pondering what more it could do to support Syrian government and Kurdish forces against Islamic State militants.

"We are mulling over what we would really do extra in order to support those who are in the battlefield, resisting and fighting with terrorists, IS first of all," Mr Putin said, ruling out deploying Russian ground troops.

"There is [an] opportunity to work on joint problems together," Mr Putin said of his talks with Mr Obama, which a US official described as "business-like".

A senior US official, speaking on condition of anonymity, told reporters after the meeting: "The Russians certainly understood the importance of there being a political resolution to the conflict in Syria, and there being a process that pursues a political resolution."

Clinking glasses, frosty looks

US-Russian ties have been deeply strained by Moscow's March 2014 annexation of Crimea from Ukraine and its support of pro-Russian separatists in the eastern part of the country.

Earlier in the day, relations between the two leaders appeared frosty - they clinked glasses at a lunch, but Mr Obama had a piercing look as Mr Putin smiled.

They also laid out starkly differing positions toward Mr Assad in their addresses before the annual United Nations General Assembly gathering of world leaders.

Mr Obama said he was willing to cooperate with Russia and Iran to try to end the four-year civil war in Syria, in which at least 200,000 people have died and millions have been driven from their homes. But he described Mr Assad as its chief culprit.

In contrast, Mr Putin said there was no alternative to cooperating with Mr Assad's military to fight Islamic State militants, and called for the creation of a broader international anti-terrorist coalition.

This appeal may compete with the coalition that the United States has assembled to fight IS.

"The United States is prepared to work with any nation, including Russia and Iran, to resolve the conflict," Mr Obama, who spoke before Putin, told the world body.

"But we must recognise that there cannot be, after so much bloodshed, so much carnage, a return to the pre-war status quo."

Obama: No role for tyrants

Mr Obama did not explicitly call for Mr Assad's ouster, and he suggested there could be a "managed transition" away from the Syrian president's rule, the latest sign that despite US animus toward Mr Assad, it was willing to see him stay for some period of time.

Mr Obama dismissed the argument that authoritarianism was the only way to combat groups such as Islamic State, saying: "In accordance with this logic, we should support tyrants like Bashar al-Assad, who drops barrel bombs to massacre innocent children, because the alternative is surely worse."

Mr Putin differed, suggesting there was no option but to work with Mr Assad, a longtime ally of Russia.

"We think it is an enormous mistake to refuse to cooperate with the Syrian government and its armed forces who are valiantly fighting terrorism face-to-face," Mr Putin said in his speech.

"We should finally acknowledge that no-one but President Assad's armed forces and [Kurdish] militia are truly fighting the Islamic State and other terrorist organisations in Syria," he said.

French president Francois Hollande and Turkish prime minister Ahmet Davutoglu both rejected the possibility of allowing Mr Assad to stay.

Bitter pill for US to follow

Russian president Vladimir Putin addresses the UN General Assembly for first time in a decade. ( AFP: Spencer Platt )

In voicing a willingness to deal with Iran and Russia, both backers of Mr Assad, Mr Obama was openly acknowledging their influence in Syria and swallowing a somewhat bitter pill for the United States.

Tehran has armed the Syrian government and, through its backing of Lebanese Hezbollah fighters, has helped Mr Assad combat rebels seeking to end his family's four-decade rule.

US officials say they believe Mr Putin's buildup of Russian forces in Syria mainly reflects Moscow's fear that Mr Assad's grip might be weakening and a desire to shore him up to retain Russian influence in the region.

They also see it as a way for Mr Putin to try to project Russian influence more widely, a goal he appeared to achieve at the weekend with Iraq's announcement that Russia, Iran, Syria and the Iraqi government were sharing intelligence on Syria.

Despite their differences over Syria and Ukraine, the US and Russia have worked with Britain, China, France and Germany, a group known as the P5+1, to secure a nuclear deal with Iran this summer.

The same group could also take up the issue of Syria, the European Union's foreign policy chief, Federica Mogherini, said after a meeting of the six foreign ministers and Iran.

But with their military response to IS making little headway, Western powers have allowed the Syrian president's backers to present him as the only practical option for a return to stability.

"If we are to succeed in fighting terrorism, the government in Damascus cannot be weakened. It must be able to carry on the fight," Iran's president Hassan Rouhani told the general assembly.

"If the Syrian government is taken out of the equation, the terrorists will enter Damascus" and "the whole country will become controlled territory, a safe haven for terrorists."

Reuters