Opposition activists have claimed the Syrian army has intensified shelling Sunni Muslim regions in central and northern Syria after UN monitors suspended operations.
Activists said at least 50 people were killed and hundreds more were wounded in the attacks.
The monitors' decision was the clearest sign yet that a peace plan brokered by Kofi Annan had collapsed after repeated violations by Assad's forces and rebels.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.
Meanwhile, President Barack Obama will hold talks with President Vladimir Putin on the sidelines of a G20 summit in Mexico today but expectations are low that they will break a deadlock over Syria's conflict.
Russia and China, veto-wielding members of the UN Security Council, have shielded President Bashar al-Assad from Western-sponsored action beyond verbal condemnation of the violence.
On Saturday, chief UN monitor General Robert Mood said increased violence had forced his unarmed observers to suspend operations to oversee Mr Annan's widely ignored 12 April ceasefire.
A rise in violence over the last month, including two massacres that cost the lives of 200 Sunni men, women and children in villages near Homs and another northwestern city Hama, has prompted greater international condemnation of President Assad.
The opposition is increasingly accusing Mr Assad of waging a military campaign of ethnic cleansing in Homs to empty the city and surrounding countryside of the majority Sunni inhabitants.
Mr Assad has repeatedly said he was resisting what he described as a foreign conspiracy to divide Syria that left him no option but to use force against "terrorists".
The United Nations says Syrian forces have killed 10,000 people in the crackdown on protest against Mr Assad's rule that broke out in March last year, inspired by uprisings across the Arab world which have toppled four autocratic leaders.
Mr Assad's government says foreign-backed Islamist militants have killed at least 2,600 Syrian police and troops.