The United States declared Monday that Russia had failed to meet its side of a deal to enforce a seven-day truce in Syria, but that Washington was willing to keep working on it.
Under the terms of an agreement struck earlier this month in Geneva, the US military would set up a joint cell with Russian forces to target Syrian jihadists if the ceasefire held.
US Secretary of State John Kerry told reporters: "We have not had seven days of calm and of delivery of humanitarian goods."
Under the terms of the Geneva agreement, the US was supposed to rein in opposition forces and Moscow was to ensure its ally Syrian leader Bashar al-Assad halt attacks.
Kerry had earlier said that this ceasefire was "holding but fragile" but that the other half of the agreement -- that Assad allow UN aid convoys to besieged areas -- was incomplete.
The Syrian military, meanwhile, has announced that after seven days it was ending its participation in the ceasefire, blaming the rebels for repeated breaches of the truce.
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Kerry, who was meeting with Palestinian leader Mahmud Abbas on the sidelines of the UN General Assembly, reacted testily to the declaration, but implied there was time to save the deal.
"It would be good if they didn't talk first to the press but if they talked to the people who are actually negotiating this," he said, in brief remarks to reporters.
"And I think it's, as I said yesterday, time to end the grandstanding and time to do the real work of delivering on the humanitarian goods that are necessary for access.
"So we just began today to see real movement of humanitarian goods, and let's see where we are. We're happy to have a conversation with them," he said, of the Russian side.
Under the deal, if fighting had been reduced significantly over seven das and aid had got through, the US and Russian militaries were to have set up a joint targeting cell in Geneva.
This would enable more accurate strikes on the Islamic State group and Fateh al-Sham Front, formerly the Al Nusra Front.
US and Russian officers were meeting Monday in Geneva, but the Defense Ministry in Moscow said it would be "pointless" to continue the truce in the face of rebel violations.