Sniper fire killed six civilians in Homs on Tuesday, among at least 20 dead nationwide, monitors said, as the head of the UN advance team acknowledged the mission faced a "difficult" task shoring up the ceasefire that went into force at dawn last Thursday.
Wednesday's bombardment targeted four rebel districts of Homs -- Jurat al-Shayah, Al-Qarabis, Khaldiyeh and Bayyada, the Syrian Observatory for Human Rights said.
They are among a string of neighbourhoods of Syria's third largest city that remain outside security force control despite a massive assault on its Bab Amr district that saw hundreds killed, including Western journalists, before troops moved in on March 1.
In Idlib, a northwestern province close to the border with Turkey which is another stronghold of fighters of Free Syrian Army, regime forces killed six civilians on Tuesday, the Britain-based Observatory said.
Five bodies were found near Idlib city, the watchdog added. More were found in the Homs province town of Qusayr and near the capital.
Two civilians and three soldiers were killed by twin bombs in the Al-Shaar neighbourhood of the second-largest city Aleppo, which has been largely spared the bloodshed rocking the country, the Observatory said.
In Daraa province, south of Damascus, cradle of the uprising against President Bashar al-Assad's regime that erupted in March last year, security force fire killed three civilians, one of them a mourner attending a funeral, it added.
The latest deaths bring the overall toll from the 13-month-old revolt to more than 11,100, according to the watchdog's figures.
Colonel Ahmed Himmiche, the Moroccan heading the UN advance team that arrived in Damascus on Sunday, said the mission would move forward one step at a time in its efforts to consolidate the shaky ceasefire.
"It's a difficult mission that needs coordination and planning," he told reporters on Tuesday.
"No ceasefire, not even the beginnings of a political process -- this mission will be one of the toughest ever undertaken by the United Nations," he added.
Syria is holding up an accord with Himmiche's advance party which threatens approval for the full mission, diplomats at the world body said.
Negotiations have become deadlocked on a memorandum of understanding which would allow the observers to operate across the country, the diplomats added.