Syrian troops shelled a rebel-held area of Homs and sent reinforcements to border areas today.
The opposition have called for fresh protests after the United Nations accused President Bashar Assad of failing to honour a peace plan which went into effect a week ago.
The latest violence came hours after Western and Arab diplomats from the Friends of Syria group met in Paris.
At the meeting yesterday, US Secretary of State Hillary Clinton called for the UN Security Council to adopt an arms embargo and other tough measures against Syria.
Earlier, UN Secretary-General Ban Ki-moon said Syria was not honouring a ceasefire, which took effect last week, and that violence was escalating.
The ceasefire is seen as the most viable way to end the bloodshed that has killed more than 9,000 people since an uprising against Mr Assad began 13 months ago.
The Britain-based Syrian Observatory for Human Rights said a mortar round was hitting the rebel-held district of Khaldiyeh in Homs every five minutes.
Citing its network of sources on the ground, the group said explosions and cracks of gunfire also rang out in the town of Qusair, near the border with Lebanon. Activists said regime forces were sending reinforcements to Qusair.
"Regime forces are fortifying their positions in eastern and western Qusair", about seven miles (10km) from Lebanon, said the head of the Observatory, Rami Abdul-Rahman.
Activists called for anti-government protests after Friday prayers.
The UN has sent a team of seven international observers into Syria, with the hopes of boosting the numbers soon.
Mr Ban has recommended the Security Council quickly approve a 300-member UN observer mission to Syria, a number larger than originally envisioned. But he said he will review ground developments before deciding when to deploy the mission.
An amateur video posted online by activists showed at least two of the observers, including the team's head, Colonel Ahmed Himiche, standing outside a UN vehicle yesterday as dozens chanted "Death is better than humiliation!" and "The people want to topple the regime!"
The United Nations hopes to have 30 ceasefire monitors in Syria next week and there are already plans for the deployment of up to 300, a spokesman for international envoy Kofi Annan said.
This comes after France called on the international community to prepare for the possible failure of the increasingly fragile peace plan.
Seven observers are already on the ground and another two will arrive on Monday, with 21 more expected by the end of the week, said Ahmad Fawzi.
The preliminary agreement between Syria and the United Nations on the deployment of UN observers says they will have freedom to go anywhere in the country by foot or by car, take pictures, and use technical equipment to monitor compliance with the ceasefire engineered by Mr Annan.
The issue of using helicopters and aircraft remains under discussion.
The larger contingent of up to 300 observers still needs to be approved by the UN Security Council, Mr Fawzi said.
In France, foreign minister Alain Juppe called on the international community to live up to its responsibilities and warned that if Mr Annan’s peace plan “doesn’t function, we have to envisage other methods”.
Mr Juppe said on France’s BFM television that his country would support a US-backed proposal for a UN arms embargo and other tough measures against Syria.
The peace plan is “the last chance before civil war... We don’t have the right to wait,” he said.
Mr Juppe hosted US Secretary of State Hillary Clinton and other diplomats in Paris on Thursday to try to work out options for Syria. The UN chief accused Syrian president Bashar Assad of failing to honour the peace plan that went into effect a week ago.
Mr Assad’s government signed the monitor agreement document in Damascus, UN Secretary-General Ban Ki-moon told the Security Council on Thursday.
The UN chief said the situation remains “highly precarious,” citing an escalation of violence including “shelling of civilian areas, grave abuses by government forces and attacks by armed groups”.
The observers will have freedom to install temporary observation posts in cities and towns, freedom to monitor military convoys approaching population centres, freedom to investigate any potential violation and freedom to access detention centres and medical centres in coordination with the International Committee of the Red Cross and Syrian authorities, the agreement says.