Government tanks have shelled two suburbs of the Syrian capital Damascus, opposition activists say.
The military assault on Hasrata and Irbin began in the early hours of the morning, after rebels attacked a government intelligence compound, they said.
There are reports of army offensives in other parts of Syria.
UN Secretary General Ban Ki-moon warned of serious repercussions if the crisis in Syria is not resolved.
Speaking at a conference in Indonesia he warned the world could not afford to look away.
"We all have a responsibility to work for a resolution of this profound and extremely dangerous situation, a crisis that has potentially massive repercussions for the region and the world," he said.
The UN Security Council is due to meet in New York to try to agree a statement on a peace plan put forward by UN and Arab League special envoy Kofi Annan.
If agreed, a statement would not be binding and would fall short of a formal resolution.
Mr Annan has spent the last few weeks meeting all sides in the conflict - putting forward proposals to try and bring about an immediate ceasefire by both sides, access for humanitarian aid and the beginning of political dialogue.
'Lots of mistakes'
The British-based Syrian Observatory for Human Rights gave details of army activity in the Khalidiya district of Homs and activists have also reported shelling in Rastan, north of Homs, and at Qalat Mudiq, northwest of Hama.
Syria restricts access to foreign media which often makes it impossible to independently verify reports coming out of the country.
The UN says more than 8,000 people have been killed in the year-long uprising, while tens of thousands of people have fled their homes.
On Tuesday Russia warned Syria's leadership it was making "a lot of mistakes", signalling Moscow may be hardening its stance on Damascus.
Russia is a key ally of Syria and, along with China, has twice thwarted attempts to agree to a UN resolution condemning Mr Assad's actions.
But observers believe Moscow's patience with Damascus has been wearing thin.
Meanwhile the US campaign group Human Rights Watch (HRW) accused elements of Syria's armed opposition of carrying out serious human rights abuses, including kidnapping, torture and execution.
The opposition Syrian National Council said in a statement that it "deplores the reported incidents of human rights violations by armed opposition groups in Syria" and it is working to ensure "abuse does not happen in the fight for freedom".
HRW has frequently accused Syria's government of abuse over the past year of conflict.