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

With just months left in office, the besiegement 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 reexamined 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 Defense, 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 300,000 people.

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 America's strategic interest.

Instead he has instead backed diplomacy as the only way out of the crisis.

But since a US-brokered ceasefire crashed on takeoff last week, Russia and Syria have launched rolling airstrikes 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.

Already the situation is being compared to Guernica -- a savage bombardment immortalized by Pablo Picasso's painting.

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

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 American 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. 

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

It has also created a refugee crisis that has destabilized 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."

Source: AFP