The United States has warned Syria to co-operate with a UN-backed peace plan.
Secretary of State Hillary Clinton urged President Bashar al-Assad to "take this path, commit to it, or face increasing pressure and isolation".
The peace plan, put forward by UN envoy Kofi Annan, has now been endorsed by the UN Security Council, with support from Russia and China.
Meanwhile, government tanks shelled two suburbs of the Syrian capital Damascus, according to opposition activists.
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.
A UN Security Council statement on Wednesday expressed full support for Mr Annan's plan to end to all violence, secure humanitarian access and to facilitate a political transition.
It also says the council will "consider further steps as appropriate", without specifying a time frame.
It is not binding and falls short of a formal resolution.
But the BBC's Nada Tawfik at the UN says diplomats hope it will intensify pressure on Mr Assad to work co-operatively with Mr Annan.
Diplomats said Western powers had agreed to soften the statement in order to gain the support of Russia and China, which had threatened to veto an earlier, tougher draft.
China and Russia have in the past blocked two resolutions by the council condemning Mr Assad's actions.
Mr Annan has spent the last few weeks meeting all sides in the conflict - putting forward proposals to try to bring about an immediate ceasefire by both sides, access for humanitarian aid and the beginning of political dialogue.
UN Secretary General Ban Ki-moon earlier 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.
In Syria itself, the British-based Syrian Observatory for Human Rights said the army had been active in the Khalidiya district of Homs. 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 - a key ally of Damascus - warned Syria's leadership it was making "a lot of mistakes", signalling Moscow may be hardening its stance on Damascus.
The same day 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 abuses during the conflict.