Activists say Syrian government forces heavily shelled residential neighbourhoods dominated by opposition fighters in the central city of Homs, just hours before the arrival in Damascus of an advance team of UN observers.
The reported shelling on Sunday is threatening the truce to which President Bashar al-Assad and rebels fighting to topple him had agreed.
Both sides accuse each other of violating the truce at the centre of the peace plan brokered by Kofi Annan, the UN-Arab League envoy.
Annan's spokesperson, Ahmad Fawzi, said the group of six observers were due to arrive on Sunday night and would be "on the ground in blue helmets tomorrow".
Meanwhile, activists told Al Jazeera that Syrian security forces are also shelling the village of Kherbet al-Jouz in Idlib, where there is a heavy presence of the Free Syria Army.
There were also reports of gunfire near the Turkish border but it was not clear which side was responsible for the gunfire.
The Reuters news agency reported an attack on a police station in Aleppo that it said was staged by opposition fighters.
The Syrian government restricts access of foreign observers, including journalists, making it difficult to verify reports of violence independently.
A Syrian security source accused "terrorist groups" of being behind new attacks, saying assaults had increased since the ceasefire to end violence was announced last week, according to the official SANA news agency.
"Since the announcement of an end to military operations, terrorist attacks have increased by dozens, causing a large loss of life," SANA said, adding "armed terrorists" killed five people in ambushes around the country on Saturday.
The UN observers were deployed after the Security Council in New York voted on Saturday to authorise an advance team of observers to help maintain Syria's ceasefire.
Saturday's resolution gave the 15-nation Security Council its first united front since the uprising against Assad began 13 months ago.
It called for immediate deployment of up to 30 monitors, to be followed by a larger contingent of up to 250 once the situation has stabilised.
Fawzi said the council would be asked to approve a full mission of about 250 observers, assuming the ceasefire holds, based on a report by Ban Ki-moon, the UN secretary-general, next week.
"What ceasefire? There's an explosion every five to six minutes,'' said Yazan, a Homs-based activist, contacted via Skype by the Associated Press news agency.
"I can also hear the sound of a reconnaissance plane. It's flying very low.''
In an amateur video posted on the internet by activists on Sunday, explosions and gunfire can be heard echoing as al-Khalidiya's skyline is engulfed in grey smoke.
|Homs-based activist Yazan said 'shelling resumed Friday and it has been escalating since then' [YouTube]
Homs-based activists said other districts including Bayada, Jurat al-Shayah, al-Qarabis and al-Qusour were also being bombarded.
"If you saw Homs right now you wouldn't recognise it,'' said Yazan, describing rubble-strewn roads and badly damaged apartment blocs.
"You walk around and it's not unusual to find dead people in cars on the street,'' he said, giving only his first name for fear of retribution.
The Local Co-ordination Committees activist network said the day started with a barrage of shells that fell at the rate of six each minute, shaking the neighbourhood of al-Khalidiya for the second consecutive day.
There was no immediate word on casualties.
Syrian troops shelled residential neighbourhoods of Homs on Saturday in the first use of heavy weapons since the ceasefire officially took place on Thursday, activists said.
The ceasefire is aimed at ending the bloodshed that has killed over 9,000 people, according to the UN. It also aims to launch inclusive Syrian-led talks on the country's political future.
Western powers and opposition leaders remain sceptical about Assad's willingness to ease his tight grip on the country, ruled by his family for four decades.
The Syrian government appears to have complied with parts of the Annan plan, while flouting others.
With the main exception of Homs, the military has largely halted random shelling and mortar attacks on rebel-held residential areas, which were the daily norm in recent weeks.
However, it has maintained an intimidating presence of troops, tanks and plainclothes security agents in the streets and demanded that anti-government protesters seek permits, despite Annan's demand that peaceful gatherings be allowed.
Activist Yazan said on Thursday, the day the ceasefire went into effect, was the only quiet day. "But the shelling resumed Friday and it has been escalating since then," the activist said.