Bombardment of the northern city of Aleppo shows no sign of ending even as the Syrian military extends a unilateral ceasefire around Damascus and opposition strongholds in the capital's eastern suburbs for another 48 hours, Al Jazeera reported.
Monday's announcement of the truce extension came as a humanitarian convoy delivered aid to 12,000 families trapped in a government-besieged area in central Syria.
The Aleppo fighting threatens to wreck the first peace talks involving the rival parties in Geneva which are due to resume at an unspecified date after breaking up in April when the opposition delegation walked out in anger.
Between 350,000 and 400,000 people are believed to remain in rebel-held parts of what was once a city of two million.
Meanwhile, in Geneva, John Kerry, the US secretary of state, said "several proposals", aimed at finding a way to restore at least a partial truce in Syria, were being discussed.
"We're getting closer to a place of understanding, but we have some work to do, and that's why we're here," he said at the start of a meeting on Monday with Adel al-Jubeir, Saudi Arabia's foreign minister.
After meeting Jubeir and Staffan de Mistura, the UN envoy for Syria, Kerry said that he hoped for more clarity in the next day or so on restoring the nationwide ceasefire.
"What is happening in Aleppo is an outrage. It's a violation of all humanitarian laws. It's a crime," Jubeir said.
"It's a violation of all the understandings that were reached."
The United States and Russia had agreed to keep extra staff in Geneva to work on the ceasefire.
"Both sides, the opposition and the regime, have contributed to this chaos, and we are working intensely in order to try to restore the cessation of hostilities," Kerry said.
De Mistura said that he would travel to Moscow for talks.
Those peace talks failed to make any headway, although de Mistura has said he hopes they can resume "during the course of May".
On Monday, France also called for a ministerial meeting of the international group supporting Syria to "restore the ceasefire".