As the violence raged, new international peace envoy Lakhdar Brahimi said change in Syria was "unavoidable", although he carefully refrained from calling for President Bashar al-Assad to step down, as his predecessor Kofi Annan had.
Speaking to the BBC, the former Algerian foreign minister admitted that his new mission was "nearly impossible".
He said he was "scared of the weight of responsibility. People are already saying people are dying and what are you doing?"
Jihad Makdissi, a spokesperson for Assad's embattled government, announced Brahimi would "soon" travel to Damascus, expressing confidence that "he will listen to us".
The Syrian Observatory for Human Rights said more than 20 people were killed in an army offensive on Al-Fan village in Hama province, one of the main arenas of conflict in the more than 17-month uprising.
The Britain-based watchdog had no details on whether those killed were civilians or rebel fighters, "but all 21 of the dead were men", said its director Rami Abdel Rahman.
Horrific images shot and posted on YouTube by activists in Al-Fan showed a long row of bodies shrouded in white cloths, laid out on the ground surrounded by scores of weeping men, women and children.
State news agency SANA said all of those killed during the Al-Fan clashes were from "an armed terrorist group that was attacking citizens and security forces".
They were among at least 103 people killed in violence across the country on Sunday, including 66 civilians, according to the latest Observatory figures.
In Damascus, twin bombs exploded near a tightly guarded government compound in the heart of the capital, wounding four people a day after a bombing killed 15 people in the city's south, state television said.
Arab monarchies in the Gulf on Sunday lambasted Syria's regime for deploying heavy weapons against its own civilians.
The six members of the Gulf Cooperation Council at a meeting in Jeddah also urged the international community to "assume their responsibilities and take measures to protect civilians" in Syria where, according to the watchdog group, more than 26 000 people have been killed in the revolt that erupted in March 2011.
The GCC – which includes Saudi Arabia, Bahrain, the United Arab Emirates, Oman, Qatar and Kuwait – in a statement issued after the meeting in the Saudi city condemned "the ongoing massacres which are due to the obstinacy of the regime in using heavy weapons, including planes and tanks" against civilians.
The latest bombings struck in Abu Remmaneh district where security buildings and the office of Vice President Faruq al-Shara are located, the television said, blaming "terrorists".
Shara is the highest-ranking Sunni Muslim in Assad's minority Alawite-led government and was the subject of repeated defection rumours last month before he made a public appearance during a visit by a senior envoy from key ally Iran.
The Ahfad al-Rasul (Grandchildren of the Prophet) brigade of the rebel Free Syrian Army claimed responsibility for the attack, also threatening to target Assad's palace.
Near Aleppo, another key battleground in the conflict, an AFP correspondent said air strikes were carried out on a residential area of Al-Bab, a rebel rear base north of the commercial hub.
Syrian rebels also laid siege to Harem, on the Turkish border, confronting the army and security forces positioned in government buildings and the town's old castle, an AFP correspondent reported.
Rights watchdogs have expressed growing concern about deteriorating conditions in areas under a protracted army siege, including parts of Aleppo and third city Homs.
Brahimi, who took over as international peace envoy on Saturday, told Al-Jazeera that "change is necessary, indispensable, unavoidable".
He said his position was to engage all the parties and he would not be drawn on Assad's future.
"It is too early to speak about who should go and who should stay," he told the channel.
"This is not a step backwards. Mr Assad is there and is the president of the present government," he said. "Kofi talked to him, and I will talk to him."
Similar comments by the veteran Algerian diplomat soon after his nomination last month drew an angry reaction from Syria's exiled opposition, which accused him of giving Assad's regime a "licence to kill".
Brahimi, who will formally take the reins in a handover ceremony with Annan and UN chief Ban Ki-moon on Tuesday, has been holding meetings at the UN headquarters in New York.
He takes over amid mounting pessimism about the prospects for peace after Annan announced he was stepping down last month, blaming divisions in the international community for the failure of his April six-point peace plan. – Sapa-AFP