Syrian troops killed 28 civilians in the city of Hama, monitors said, as UN military observers toured protest centres near the capital, and both Brussels and Washington imposed new sanctions.
The persistent bloodshed 11 days into a ceasefire sparked growing criticism from opposition activists of the fledgling UN mission, which still numbers just eight observers out of a planned initial deployment of 30.
Despite concerns over the mounting violence that the UN says has left over 9,000 people dead in 13 months of fighting, UN leader Ban Ki-moon gave the go-ahead for the deployment of 300 ceasefire monitors to Syria from next week.
Ban insisted Monday that the government President Bashar al-Assad ensure the protection of the unarmed observers and allow them to travel freely throughout the country.
His political chief B. Lynn Pascoe however told the UN Security Council that Assad's compliance with a ceasefire plan "remains incomplete."
Government troops strafed the Arbaeen neighbourhood of the central city of Hama and its environs on Monday with light and heavy machine guns, said the Syrian Observatory for Human Rights in reporting the 28 civilian deaths.
Video footage posted online by activists showed mortar rounds hitting the area, with plumes of smoke rising to the sky.
The UN observers visited several rebel suburbs near the capital and were met by thousands of protesters demanding the collapse of the regime.
Monitors also visited the town of Zabadani, where regime forces and rebel fighters have clashed repeatedly in past months.
Activists' videos showed monitors passing by army tanks posted in the streets, despite a call within UN-Arab League envoy Kofi Annan's six-point peace plan for the withdrawal of armour from residential areas.
However, Deputy Foreign Minister Faisal Meqdad stressed his government's "total commitment to respecting the Annan plan," adding that the "armed terrorist groups" -- a reference to the rebels -- had not yet accepted it.
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Arab League chief Nabil al-Arabi called for a political process to resolve the crisis.
"A political solution cannot be considered separately from today's goal of achieving a ceasefire," Arabi told reporters.
Two members of the advance team of observers set up base Sunday in the central city of Homs, scene of some of the fiercest fighting between troops and rebels since the outbreak of the revolt against Assad.
State news agency SANA said the military observers sent to monitor the April 12 ceasefire toured the battered city's Al-Waer neighbourhood.
Activists have been sceptical of the UN mission, saying the regime was simply buying time and was not committed to the ceasefire plan.
Despite a lull in the fighting in regions visited by the observers, the violence has continued unabated in other areas, including Damascus, Hama and Daraa provinces, they say.
In a sign of Western frustration with Damascus, the European Union agreed to slap new sanctions on the regime, banning luxury goods exports and further restricting the sale of items used to repress dissidents.
Brussels also expanded the blacklist of dual-use goods that can be used to crack down on the population or manufacture equipment used for internal repression.
US President Barack Obama ordered sanctions and visa bans for companies and individuals providing technological know-how, computers or other equipment that help Syria and its main regional ally Iran oppress their people.
Obama said in an executive order that the two nations had committed serious human rights abuses through network disruption and by using tracking technology and by perpetrating the "malign use of technology."
A Muslim charity, meanwhile, said that around 300 Syrian refugees fleeing to Jordan had escaped a Syrian army ambush near the border but more than 30 of them had been wounded by gunfire.
"The Jordanian army switched off the lights on the border when the shooting started to prevent snipers from killing the refugees," Zayed Hammad, head of the Ketab and Sunna Society, told AFP. One of the wounded had since died.
In Lebanon, another neighbour, the national news agency NNA said two Lebanese and a Syrian national were arrested with a van loaded with arms and rockets headed in the direction of the border with Syria.