Syrian troops have shelled a rebel-held area of Homs and sent reinforcements to border areas as the opposition 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 on Thursday, 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 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 as dozens chanted "Death is better than humiliation!" and "The people want to topple the regime!"