Syrian regime forces on Monday announced an end to the week-long ceasefire brokered by the United States and Russia, blaming opposition groups for its failure.
"Syria's army announces the end of the freeze on fighting that began at 7:00 pm (1600 GMT) on September 12, 2016 in accordance with the US-Russia agreement," the statement carried by state news agency SANA said.
The truce "was supposed to be a real chance to stop the bloodshed, but the armed terrorist groups flouted this agreement," the statement said.
It accused opposition groups of carrying out more than 300 truce violations across the country and of failing to "commit to a single element" of the deal.
However, artillery shelling and airstrikes hit opposition-held parts of Aleppo, correspondents in the city reported following the regime's statement. The raids hit Sukkari and Amiriyah, two eastern neighborhoods of the battleground city.
The Syrian Observatory for Human Rights, a Britain-based monitoring group, reported that shells were raining down on the northern city while a journalist said four huge explosions were heard in the war-torn city.
Syrian or Russian warplanes bombed the opposition-held areas in the city of Aleppo and nearby villages, the observatory said, reporting a number of dead and injured.
The monitoring group said it was not clear if the jets were Syrian or Russian. Moscow supports Bashar Assad with its air force.
The raids came as what is likely to be the final attempt by the U.S. administration of President Barack Obama to find a negotiated solution to the five year old civil war appeared close to collapse.
U.S. Secretary of State John Kerry said it was too early to call the ceasefire finished, and the United Nations said that only Washington and Moscow could declare it over, as they were the ones who had originally agreed it.
Washington said it was working to extend the truce but called on Russia to first clarify the Syrian army's statement that it was over.
Russian and U.S. officials met in Geneva on Monday and the International Syria Support Group - the countries backing the Syria peace process - were scheduled to meet on Tuesday in New York to assess the ceasefire agreement.
But both the Syrian army and the opposition spoke openly of returning to the battlefield.
Kerry told reporters in New York that the seven days of calm and aid deliveries envisaged in the truce had not yet taken place.
"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," Kerry said. "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."
Aid was delivered to the besieged town of Talbiseh in Homs province on Monday, the Red Cross said, for the first time since July. The convoy brought in food, water and hygiene supplies for up to 84,000 people, it said.
"I am pained and disappointed that a United Nations convoy has yet to cross into Syria from Turkey, and safely reach eastern Aleppo," the U.N. Under Secretary General for Humanitarian Affairs Stephen O'Brien said in a statement.
The United Nations said it had received regime approval to reach nearly all the besieged and hard-to-reach areas where it sought to bring aid, but access to many areas was still constrained by fighting, insecurity and administrative delays.
A military source had earlier told AFP that the ceasefire deal was set to expire at 7:00 pm (1600 GMT) on Monday evening.
The agreement, negotiated by top diplomats from government backer Russia and the United States, saw an initial drop in fighting across the country after it came into force on September 12.
But violence began to escalate late last week.
Already widely violated since it took effect, the ceasefire came under added strain at the weekend when Russia said jets from the U.S.-led coalition against Daesh killed more than 60 regime soldiers in eastern Syria.
Assad called that incident "flagrant aggression". Washington has called it a mistake.