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US President Barack Obama has defended his refusal to use military force to end Syria's brutal civil war, as diplomatic efforts falter and a humanitarian crisis of historic proportions unfolds in Aleppo.

With just months left in office, the siege and bombardment of Syria's second city has put Obama's polices back under the spotlight and exposed deep unease within his administration.

"There hasn't been probably a week that's gone by in which I haven't re-examined some of the underlying premises around how we're dealing with the situation in Syria," Obama told a CNN town hall debate.

"I'll sit in the situation room with my secretary of defence, the chairman of the joint chiefs of staff, we'll bring in outside experts - I will bring in critics of my policy to find out, OK, you don't think this is the right way to go."  

But, Obama insisted, "in Syria, there is not a scenario in which - absent us deploying large numbers of troops - we can stop a civil war in which both sides are deeply dug in. 

"There are going to be some bad things that happen around the world, and we have to be judicious."

The civil war has dragged on for more than five years and so far killed at least 400,000 people.

Since a US-brokered ceasefire crashed last week, Russia and Syria have launched rolling airstrikes on rebel-held eastern Aleppo [AFP]

Instead he has backed diplomacy as the only way out of the crisis. Obama has sent around 300 troops to Syria, focused on the battle against the Islamic State group, but has refused to plunge them into a civil war that is not in US' strategic interest.

But since a US-brokered ceasefire crashed on takeoff last week, Russia and Syria have launched rolling air raids on rebel-held eastern Aleppo, where a quarter of a million people are trapped.

Forces loyal to Bashar al-Assad's regime have simultaneously launched a ground assault, eying a victory that could prove decisive in the five-year war.

On Wednesday, two of the largest hospitals in rebel-held parts of the city were bombed, prompting UN Secretary General Ban Ki-moon to describe that attack as a "war crime".

In response, Obama's administration has threatened to suspend its engagement with Russia unless the bombing stops.

Obama insists that ultimately there must be a political solution and the US would try to ameliorate the suffering in Syria.

But Obama again insisted that ultimately there must be a political solution, while saying that the US would try to ameliorate the suffering.

The state department on Wednesday said it would release a further $364 million to UN aid agencies and NGOs working to help vulnerable Syrian civilians inside and outside the war-torn country.

Diplomacy, not war

Obama came to office on a platform of opposition to the war in Iraq and ending the war in Afghanistan.

Throughout his presidency he has been reluctant to deploy combat troops and argued for a more judicious use of US military power and assessment of the national interest.

"Historically, if you look at what happens to great nations, more often than not, they end up having problems because they are overextended, don't have a clear sense of what is their core interests," Obama said.

Historically, if you look at what happens to great nations, more often than not, they end up having problems because they are overextended, don't have a clear sense of what is their core interests - Barack Obama, US President

Critics argue that he has defined the national interest too narrowly and that the Syrian conflict has called US' reputation and commitment to the rule of law into serious question.

It has also created a refugee crisis that has destabilised Europe and has allowed Russia and Iran to assert greater power in the Middle East.

"It is long past time for the United States to reassess its shameful approach to the Syrian crisis," said Charles Lister of the Middle East Institute.

"US indecision, risk aversion, a total divergence between rhetoric and policy, and a failure to uphold clearly stated 'red lines' have all combined into what can best be described as a cold-hearted, hypocritical approach.

"At worst, Washington has indirectly abetted the wholesale destruction of a nation-state, in direct contradiction to its fundamental national security interests and its most tightly held values."