Explosions rocked the capital Damascus, as witnesses said that warplanes launched their heaviest air raids yet across Syria.
A blast struck on Monday near a bakery in Jaramana, a southeastern district of Damascus controlled by forces loyal to President Bashar al-Assad, killing at least 11 people, including children and women, and injuring scores, according to both activists and state news agency SANA.
A second explosion was reported in al-Hajar al-Aswad neighbourhood.
Activists there said an airstrike by government forces struck a bus and left at least 10 people killed. Meanwhile state TV said it was a "terrorist" car bomb that left an unspecified number of victims.
Lena al-Shami, an activist in Damascus, said: "The regime is trying to cover up the massacre it caused by the shelling on the bus. So they are calling it a car bomb."
Al-Hajar al-Aswad was the scene of intense mid-summer combat between rebels and the army when fighting broke out in several districts of the Syrian capital.
Activists said that warplanes launched 60 airstrikes on Monday, the most intense air raids across the country since the uprising began 19 months ago. The suburbs of the capital Damascus were particularly hard hit.
The violence brought a bloody end to a failed attempt at a four-day ceasefire, during which 500 people were reported dead. At least 80 people were killed on Monday alone, activists said.
Lakhdar Brahimi, appointed by the Arab League and United Nations as envoy to Syria, had sought to get both the rebels and Assad troops to halt their fighting over the Muslim Eid al-Adha holiday. Instead, government jets and artillery bombarded opposition neighbourhoods and rebels launched attacks on military checkpoints.
Speaking in Moscow alongside Foreign Minister Sergei Lavrov, Brahimi said he was "terribly sorry" that the ceasefire had failed and that the conflict was getting even worse.
He said the UN "is not considering" sending an armed peacekeeping force to Syria, though relevant officials were conducting contingency planning in case the UN Security Council ordered such a mission. That is highly unlikely, with Russia and China - two Council members - opposing any kind of international intervention.
In-depth coverage of escalating violence across Syria
The Syrian army warned late Sunday night that it will strike "remnants of terrorists with an iron fist" after they "repeatedly violated the cease-fire." The regime of Assad often refers to those waging the uprising as "terrorists".
Brahimi's ceasefire was violated almost as soon as it was agreed, and both rebels and government troops initiated firefights since the three-day holiday began at sunset on October 25. He is also due to visit China.
Explosions, mortar attacks and gunfire have been heard throughout Aleppo, the country's second-largest city, where neighbourhoods have been reduced to rubble as a result of the months-long struggle there.
In the northwestern Idlib province, the attacks killed at least 16 people on Sunday, including seven children and five women, an activist group said.
"The ceasefire is practically over. Damascus has been under brutal air raids since day one and hundreds of people have been arrested," Fawaz Tello, an opposition campaigner, told the Reuters news agency.
State news agency SANA said "armed terrorist groups" attacked checkpoints and planted explosive devices in several cities during the Eid holiday.
Journalist 'under arrest'
Rebels have also kidnapped a Lebanese journalist operating in the contested north of the country for making reports the fighters deemed unhelpful to their cause.
In a video released on Sunday, Fida Itani said he was in good health but being held under house arrest by the "Northern Storm Brigade".
The rebel group's Facebook page said Itani's work was "not compatible with the revolution" and that he would be held for "a short time".
A group of Syrian activists and intellectuals condemned the arrest of Itani, saying such the accusation "brings back to memory the pretext the Syrian regime used to detain and torture thousands of Syrians".
The opposition claims at least 32,000 people have been killed since the uprising began in March last year. Hundreds of thousands of people have fled to neighbouring countries.
As winter approaches, life will get worse for displaced families inside Syria and refugees filling up temporary camps in border areas in Jordan, Turkey and Lebanon. Valerie Amos, the UN's chief humanitarian relief officer, has said that up to three million Syrians have been affected by the crisis.
In a statement released last week, she called on all parties to stop targeting civilians and said "ordinary women, men and children ... suffer from the indiscriminate use of explosive weapons like cluster munitions".