Secretary of State John F. Kerry speaks in Brussels on Oct. 4. (Emmanuel Dunand/AFP/Getty Images)

Secretary of State John F. Kerry lashed out at Russia on Tuesday after a breakdown in efforts to restore a cease-fire in Syria, accusing Moscow of favoring war over diplomacy amid widening rifts with Washington.

The condemnation left little hope that talks with the Kremlin on Syria could resume soon, while Russian and Syrian forces increased bombardment of the besieged city of Aleppo in attempts to crush one of the last major strongholds of rebel forces.

The State Department on Monday broke off months of U.S.-Russia negotiations over Syria, with violence increasing in recent weeks after the collapse of a cease-fire deal.

Speaking in Brussels ahead of a donors conference for Afghanistan, Kerry ventured no new approaches to resolving the Syrian conflict, voicing mostly bitterness that his efforts to engage Russia have brought no progress. The Kremlin is Syrian President Bashar al-Assad’s most important international backer, while the United States has supported anti-Assad rebels.

“I have to tell you with a great sense of outrage that Russia has turned a blind eye to Assad’s deplorable use of these weapons of war that he has chosen — chlorine gas, barrel bombs against his people,” Kerry said. “Together, the Syrian regime and Russia seem to have rejected diplomacy, in the furtherance of trying to pursue a military victory, over the broken bodies, the bombed-out hospitals, the traumatized children of a long-suffering land.”

In Damascus, meanwhile, state media accused rebel forces of shelling government-held neighborhoods in Aleppo.

The conflict has stretched more than five years, killing more than 400,000 people and displacing millions.

“People that are serious about pursuing peace behave differently from the way Russia has chosen how to behave,” Kerry said.

The suspension of talks “does not come lightly,” he said, condemning Russia’s alliance with Assad as “irresponsible and profoundly ill-advised.” Kerry said the United States will continue separate military contacts with Russia to enable both sides to pursue campaigns against the Islamic State militant group.

Kerry’s appeals were echoed by U.N. Secretary General Ban Ki-moon, who said he planned meetings with Kerry and Russian envoys in a bid to restart a cease-fire effort.

Kerry, who is making one of his final tours across the Atlantic as part of the Obama administration, told Europeans to hold fast to the alliances that they built out of the rubble of World War II, even as Britain negotiates a divorce from the European Union.

“As much as some of us may wish the U.K. vote had gone the other way, the lesson that we have to take from this democratic choice is not that we need less Europe, less U.K.,” Kerry said, referring to Britain’s June referendum on whether to leave the E.U. “Rather, we need more of both — more security, more prosperity, more collaboration among the U.S., the U.K. and the E.U.”

Otherwise, he said, “there are demagogues out there from the left and the right who fan the fears of change and who believe that bluster, often tinged with bigotry, can provide a pathway to their own power.”

He did not cite any specific political figures, but right-wing parties have gained ground in France, Germany and elsewhere.

Britain and the E.U. increasingly appear headed for a tough split as positions harden on both sides ahead of the start of formal negotiations, expected by the end of March.

British Prime Minister Theresa May said Sunday that she will make limiting immigration a priority in the negotiations, a stance that makes it less likely that her country will retain the full range of economic benefits it now has as an E.U. member. 

Kerry acknowledged that the Obama administration may be a bigger cheerleader for Europe than European leaders.

“I encourage you all to believe in yourselves as much as we believe in you,” he said.

Brian Murphy in Washington contributed to this report.