WASHINGTON/BEIRUT -  The Kremlin vowed on Thursday to press on with its assault in Syria, while US officials searched for a tougher response to Russia’s decision to ignore the peace process and seek military victory on behalf of President Bashar al-Assad.

Moscow and Damascus launched an assault to recapture the rebel-held sector of Aleppo this month, abandoning a new ceasefire a week after it took effect to embark on what could be the biggest battle of a nearly six-year war.

Rebel fighters have launched an advance of their own in countryside near the central city of Hama, where they said they made gains on Thursday.

The United States and European Union accuse Russia of torpedoing diplomacy to pursue military victory in Aleppo, and say Moscow and Damascus are guilty of war crimes for targeting civilians, hospitals and aid workers to break the will of 250,000 people living under siege inside Syria’s largest city. EU foreign affairs chief Federica Mogherini called the air strikes in Aleppo a “massacre” and said European governments were considering their response. Russia and the Syrian government say they are targeting only militants.

US Secretary of State John Kerry, who personally negotiated the failed truce in talks with Russia despite scepticism from other senior US officials, has said Washington could walk away from diplomacy unless the fighting stops. He has called for a halt to flights, a step rejected by Moscow. Kremlin spokesman Dmitry Peskov said on Thursday Russia would “continue the operation of its air force in support of the anti-terrorist activity of Syria’s armed forces”.

Peskov said Washington was to blame for the fighting, by failing to meet an obligation to separate “moderate” rebel fighters from terrorists.

“In general, we express regret at the rather non-constructive nature of the rhetoric voiced by Washington in the past days.”

US officials are considering tougher responses to the Russian-backed Syrian government assault, including military options, although they have described the range of possible responses as limited and say risky measures like air strikes on Syrian targets or sending US jets to escort aid are unlikely.

Recapturing Aleppo would be the biggest victory of the war for government forces, and a potential turning point in a conflict that until now most outside countries had said would never be won by force.

The multi-sided civil war has killed hundreds of thousands of people, made half of Syrians homeless, and allowed much of the east of the country to fall into the hands of Islamic State militants who are enemies of all other sides.

Aleppo has been divided into government and opposition sectors for four years, and its rebel zone is now the only major urban area still in the hands of anti-Assad fighters supported by the West and Arab states. The government lay siege to it in July, cutting off those trapped inside from food and medicine.

The last week of bombing has killed hundreds of people and left many hundreds more wounded, with no way to bring in medical supplies. There are only around 30 doctors inside the besieged zone. The two biggest hospitals were knocked out of service by air strikes or shelling on Wednesday. Russia says the only way to defeat Islamic State is to support Assad. Washington says the Syrian president has too much blood on his hands and must leave power.

Meanwhile, 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.”