Brussels - US Secretary of State John Kerry insisted on Tuesday that Washington was still seeking peace in Syria, but said it had dropped ceasefire talks with Russia because of "outrage' over Moscow's support for the regime.
Kerry accused Moscow of turning a "blind eye" to the use by President Bashar Assad's forces of weapons including chlorine gas against rebels and civilians in Syria.
"I want to be very, very clear to everybody, we are not giving up on the Syrian people, we are not abandoning the pursuit of peace," Kerry said in a speech in the Belgian capital Brussels.
"We are not going to leave the multilateral field, we are going to continue to try to find a way forward in order to end this war", including through the United Nations or through smaller groups of countries.
The United States on Monday said it had suspended two-way talks with Moscow on reviving a truce in Syria as Russian-backed government forces pressed on with an onslaught on rebel-held parts of the city of Aleppo.
"Believe me, we did not come lightly" to the decision to abandon the talks with Russia, Kerry said.
He launched a withering attack on Moscow for its "irresponsible and profoundly ill-advised decision" to support Assad.
"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 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 furtherance of trying to pursue a military victory over the broken bodies, bombed-out hospitals, the traumatised children of a long-suffering land.
"People who are serious about making peace behave differently from the way Russia has chosen to behave."
He added, however, that the United States and Russia had not suspended "deconfliction" communications between their respective militaries in the fight against the Islamic State jihadist group.
Russia did not immediately react to Kerry's comments.
But Moscow blamed the United States for the agreement's collapse, saying Washington had never been able to separate rebels on the ground from the jihadist group Fateh al-Sham Front, formerly al-Qaeda's Syrian affiliate.