WASHINGTON/BEIRUT (Reuters) - The United States is close to suspending talks with Russia on a ceasefire in Syria, U.S. Secretary of State John Kerry said on Thursday, as the Kremlin vowed to press on with an assault on the city of Aleppo.

A boy walks amid damaged buildings in the rebel held area of al-Kalaseh neighbourhood of Aleppo, Syria, September 29, 2016. REUTERS/Abdalrhman Ismail

Moscow and Damascus launched a campaign to recapture the rebel-held sector of Syria’s biggest city this month, abandoning a ceasefire a week after it took effect to embark on what could be the biggest battle of a nearly six-year war.

Syrian government forces made a significant advance, capturing the Handarat refugee camp a few kilometers (miles) north of the city. They had briefly seized it on Saturday, before losing it again in a rebel counter attack.

Rebel fighters have launched an advance of their own 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 targeting civilians, hospitals and aid workers to break the will of 250,000 people living under siege in the city.

The United States called the assault on Aleppo by Syria and Russia “a gift” to Islamic State on Thursday, saying it was sowing doom and would generate more recruits for the militant group.

Syria’s U.N. Ambassador Bashar Ja’afari rejected accusations that his government was killing civilians.

But U.S. officials are searching for a tougher response to Russia’s decision to ignore the peace process and seek military victory on behalf of President Bashar al-Assad.

“We are on the verge of suspending the discussion because it is irrational in the context of the kind of bombing taking place to be sitting there trying to take things seriously,” Kerry told a public policy conference in Washington.

“It is one of those moments where we are going to have to pursue other alternatives,” he added.

Kerry spoke to Russian Foreign Minister Sergei Lavrov on Thursday, the U.S. State Department said but it did not announce a suspension of the diplomacy, suggesting Washington may give Moscow a little more time.

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 the Syrian population homeless, and allowed much of the east of the country to fall into the hands of Islamic State jihadists who are enemies of all other sides.

EU CONDEMNS ALEPPO ‘MASSACRE’

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.

Deputy Secretary of State Antony Blinken told U.S. lawmakers President Barack Obama had asked staff to look at how Washington might respond.

“The president has asked all of the agencies to put forward options, some familiar, some new, that we are very actively reviewing,” Blinken said, adding that officials would “work through these in the days ahead.”

Kremlin spokesman Dmitry Peskov, meanwhile, said 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 those he called terrorists.

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

U.S. officials say they 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 U.S. jets to escort aid are unlikely.

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Two U.S. officials said the speed with which the diplomatic track collapsed in Syria and pro-government forces advanced in Aleppo had caught some in the administration off guard.

Possible responses include allowing Gulf allies to supply rebels with more sophisticated weapons, or carrying out a U.S. air strike on a Syrian government air base, viewed as less likely because of the potential for causing Russian casualties, the officials told Reuters, speaking on condition of anonymity.

One of the officials said the list of options included supporting rebel counter-attacks elsewhere with additional weaponry or even air strikes, which “might not reverse the tide of battle, but might cause the Russians to stop and think”.

BATTLE FOR ALEPPO

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 laid 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 wounded many hundreds more, with no way to bring in medical supplies. There are only around 30 doctors inside the besieged zone, and eastern Aleppo’s two biggest hospitals were knocked out of service by air strikes or shelling on Wednesday.

Bouthaina Shaaban, an adviser to Assad, denied Syrian planes had bombed the hospitals, saying the question was “insulting”.

“What is the interest of the Syrian government to bomb its own hospitals?” she said on Australia’s ABC TV. “This is not the first time that such an allegation is uttered and then proven to be absolutely false.”

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.

Washington is bombing Islamic State in the east but has avoided direct participation in the civil war in the rest of the country, leaving the field open to Russia, which joined the war a year ago tipping the conflict in favor of its ally Assad.

FEROCITY OF ASSAULT

The ferocity of the assault on Aleppo is driving many of the Western-backed anti-Assad groups to cooperate more closely with jihadist fighters, the opposite of the strategy Washington had hoped to pursue, rebel officials told Reuters.

In Aleppo, rebels in the Free Syrian Army are sharing operational planning with Jaish al-Fatah, an alliance of Islamist groups that includes the former Syrian wing of al Qaeda.

Meanwhile, in nearby Hama province, FSA groups armed with U.S.-made anti-tank missiles are taking part in a major offensive with the al Qaeda-inspired Jund al-Aqsa group.

The FSA rebels have deep ideological differences with the jihadists, and have even fought them at times, but say survival is the main consideration.

“At a time when we are dying, it is not logical to first check if a group is classified as terrorist or not before cooperating with it,” said a senior official in one of the Aleppo-based rebel factions. “The only option you have is to go in this direction.”