The suicide bombing that targeted a building used by pro-regime militiamen in Salmiyeh, a town in the central province of Hama, killed more than 30 people, the Syrian Observatory for Human Rights said.
State news agency SANA also reported the blast, saying that "a terrorist suicide car bomb was detonated in the heart of Salmiyeh, leaving a number of people killed and others wounded".
The Britain-based Observatory simultaneously reported a deadly powerful explosion in Damascus's upscale Dumar neighbourhood, but gave no further details and was unable to provide an immediate death toll.
The blasts came as Arab League chief Nabil al-Arabi said the mission of the international peace envoy to Syria, Lakhdar Brahimi, has so far not even "yielded a glimmer of hope" to end the 22-month conflict.
The head of the 22-member bloc urged the Arab leaders to call "the UN Security Council for an immediate meeting and to issue a resolution enforcing a ceasefire to stop the bloodbath".
He also called for an "international monitoring force to make sure that fighting has stopped".
More than 60,000 people have been killed in the conflict that erupted in March 2011 as a popular uprising against the Assad regime, according to the United Nations.
The UN mission assessing the "enormous" humanitarian needs in Syria found people -- especially children -- in dire need of food, medical care and clean water, and said it would conduct a major humanitarian operation.
Martin Nesirky, spokesman for UN Secretary General Ban Ki-moon, said a team from seven humanitarian agencies visited the city of Homs and on Monday morning crossed conflict lines into Talbiyeh.
"It has to be a big UN humanitarian operation in Syria. That is what the people expect of this mission," said John Ging, director of operations for the Office for the Coordination of Humanitarian Affairs, who headed the team.
About four million Syrians, half of them driven from their homes by the fighting, are in urgent need of aid, the UN says.
Meanwhile staunch Assad ally Moscow, which has repeatedly vetoed UN resolutions to impose sanctions on Damascus, said it would send two planes to help evacuate Russian citizens from Syria via Lebanon.
Russia "will send two planes to Beirut in Lebanon so all the Russians who wish to can leave Syria," Irina Rossious, spokeswoman for the emergency situations ministry, was quoted as saying by Interfax news agency.
"More than 100 Russians are expected to leave Syria on board these planes," she said, without giving any more details.
On the ground, fierce fighting raged between rebels and forces loyal to Assad, including militias, as the Observatory reported the formation of a new paramilitary force of men and women, some trained by key ally Iran, to fight what is now becoming a guerrilla war.
The Observatory, which relies on activists and medics on the ground for its information, said the National Defence Army gathers together existing popular committees of pro-regime civilian fighters under a new better-trained and armed hierarchy.
The Observatory gave an initial toll of 142 people killed nationwide on Monday, including 34 civilians and the 30 killed in the Hama car bombing.