Syrian forces were locked in fierce battles with opposition fighters in one city and shelled another, activists said, even as a handful of UN monitors tasked with overseeing a fragile ceasefire were due to begin work.
President Bashar al-Assad's forces killed two civilians in the central city of Hama, and were fighting rebels at Idlib in the northwest, while also shelling the flashpoint city of Homs, according to the Syrian Observatory for Human Rights on Monday.
At least 41 people, mostly civilians, have been reported killed by activists in violence since the UN-backed ceasefire came into effect on Thursday morning, prompting Ban Ki-moon, the UN secretary-general, to urge Damascus to ensure the truce does not collapse.
An advance team of five international observers arrived in Damascus late on Sunday, the United Nations said.
The delegation, the first of 30 monitors the UN Security Council approved on Saturday, will set up a headquarters and prepare routines so the mission can verify a cessation of hostilities is holding.
Annan's spokesperson said the team, led by Moroccan Colonel Ahmed Himmiche, met Syrian foreign ministry officials on Monday to discuss ground rules, including what freedom of movement the observers would have.
"We will start our mission as soon as possible and we hope it will be a success,'' Himmiche told the Associated Press news agency as he left a Damascus hotel along with his team Monday morning.
Kieran Dwyer, a UN peacekeeping department spokesperson, said further monitors would arrive in Syria in "coming days".
Louay Safi, member of opposition group, the Syrian National Council, speaks to Al Jazeera about the UN ceasefire plan
The next 25 would come from missions around the Middle East and Africa "so we can move people quickly and they are experienced in the region," he told the AFP news agency.
UN-Arab League envoy Kofi Annan wants more than 200 observers to be deployed in Syria, but the Security Council has said there would only be a full mission if the violence halted.
Syria's SANA state news agency said Syria "welcomed" the observer mission, and hoped the monitors would see for themselves the "crimes" committed by "armed terrorist groups."
The Security Council demanded full freedom of movement for the UN team, but critics of Syria say the government could obstruct its work.
The failure of an Arab League observer mission earlier this year was blamed in part on government restrictions imposed on the observers, including having to travel with government minders.
Tarek Badrakhan, an activist from the battered and almost deserted Homs district of al-Khalidiya, said the Syrian government had resumed its intense bombardment of the neighbourhood early on Monday for the third consecutive day.
"The shelling hasn't stopped for one minute since this morning. There are buildings on fire right now,'' he said via Skype.
Badrakhan and other activists said the army appeared to be on a push to take control of the last opposition-held districts in Homs, and was pounding al-Khaldiya from three sides.
He said half of the nearby district of al-Bayada fell under the army's control on Sunday night. Troops were trying to storm al-Qarabis and Jurat al-Shayah but the rebel Free Syrian Army (FSA) was repelling them, he said, referring to the group of armed opposition fighters and defectors from the Syrian army.
"We hope that the observers would come to Homs as soon as possible because if things go on like this, there won't be anything left called Homs,'' Badrakhan said.
Two activist groups, the Local Co-ordination Committees and the Syrian Observatory for Human Rights (SOHR), confirmed the intense shelling of Homs and said two people were killed in the city of Hama in central Syria on Monday when security forces opened fire on their car.
Continuing violence apparently perpetrated by Syrian government forces has raised doubts over Assad's commitment to the Annan peace plan.
While Syrian forces have mostly halted shelling of rebel-held neighbourhoods, with the exception of Homs, they have ignored calls to pull troops out of urban centres.
The United Nations estimates that more than 9,000 people have been killed in violence related to the uprising against Bashar al-Assad's government since protests began in March 2011.