John Kerry accused Russia of prolonging the conflict in Syria and turning a blind eye to the use of barrel bombs and chlorine gas, but he did not rule out a new diplomatic push to end the five-year war.

Speaking a day after Washington suspended talks with Moscow over a Syria ceasefire, the US secretary of state said Russia and the Syrian regime were pursuing a “military victory” in Syria over the “broken bodies” of children and “bombed-out hospitals”.

However, he gave no indication about how the collapse of the ceasefire agreement might impact US strategy in Syria, including the potential for stepped-up military intervention in the conflict.

The intense bombardment of Aleppo, the divided northern Syrian city, has, in effect, left the Obama administration stranded — outmanoeuvred by the Russians but with little apparent appetite to respond in kind.

For much of the last year, Mr Kerry has been an advocate for increased US military support for rebels fighting to overthrow Syrian President Bashar al-Assad, which he argued would help create leverage over Moscow and Damascus in any diplomatic process. But the argument won little support at the White House.

Instead, Mr Kerry has watched as the Syrian regime and its Russian allies have in effect used the negotiating process over a ceasefire agreement to step up their military campaign to retake Aleppo, which would be a major strategic and psychological blow to the opposition.

Russia has turned a blind eye to Assad’s deplorable use of chlorine gas and barrel bombs against his people. Together, the Syrian regime and Russia have rejected diplomacy

“We acknowledge in sorrow, and I have to tell you with a great sense of outrage, that Russia has turned a blind eye to Assad’s deplorable use of chlorine gas and barrel bombs against his people,” Mr Kerry said in a speech in Brussels on Tuesday. “Together, the Syrian regime and Russia have rejected diplomacy.”

The US and Russia agreed on the ceasefire deal in early September. But within a week it had collapsed as regime and Russian attacks on eastern Aleppo intensified, opening the prospect that the city could fall and leaving the US and its allies accusing Moscow of “war crimes.”

“Unless the balance on the ground drastically shifts, the Assad regime will eventually retake from opposition fighters all of Aleppo and the outlying districts of Damascus,” said Robert Ford, former US ambassador to Syria now at the Middle East Institute. “The Obama administration’s analysis that there is no military solution to the conflict appears incorrect.”

Rebels, however, remain optimistic about holding their territory, saying opposition forces were prepared for sustained street battles in the residential areas.

Unless the balance on the ground drastically shifts, the Assad regime will eventually retake from opposition fighters all of Aleppo and the outlying districts of Damascus

“The regime is attacking with all of its forces and capabilities,” said Yasser Alyousef, the political representative of the Nour al-Din al-Zinki brigades. “It is true that the regime and the Russians achieved gains in the geographically open areas due to the continuous strikes but this would be of no help to them in the city.”

Wissam Zaraqa, a resident of the city, said what people in Aleppo need is an end to the siege, which has caused shortages of key food items and medical supplies. About 250,000 people are trapped in Aleppo’s rebel-held east, and recent air strikes have forced the two main hospitals in the area to close.

“We are not after a new ceasefire,” said Mr Zaraqa. “We want a battle to break the siege.”

The failure of the ceasefire has reignited debate within the Obama administration about a “Plan B” for Syria, with options including stepped-up assistance to the rebel groups who have been working with the CIA in Aleppo and elsewhere.

Although the US has ruled out giving sophisticated anti-aircraft systems to the opposition forces for fear that they could fall into the hands of extremists, a US official said that Gulf allies, such as Saudi Arabia, or Turkey might now push for the rebels they support to receive such weaponry.

Yet the White House has remained reluctant to support military options that would draw the US into a more direct confrontation with Russia or the Assad regime.

Josh Earnest, White House spokesman, said that US policy would continue to give priority to the fight against jihadi groups in Syria.

“With regard to US options, we obviously have been very focused on countering ISIL [Isis] and other extremist groups that are using the chaos in Syria as a safe haven to plot and carry out attacks,” he said.

In a leaked audio of a conversation he held 10 days ago with Syrian activists in New York, Mr Kerry acknowledged that there was little political support for a bigger US military role in the country.

“I think you’re looking at three people, four people in the administration who have all argued for use of force, and I lost the argument,” he said.

Mr Kerry said on Tuesday that the Obama administration would continue to look for diplomatic paths to end the conflict.

“We have not suspended multilateral efforts through the International Syria Support Group and the UN,” he said. “We will continue to pursue a meaningful, sustainable, enforceable cessation of hostilities throughout the country — and that includes the grounding of Syrian and Russian combat aircraft in designated areas.”