Syrian troops pounded the protest hub of Homs, as an Arab League peacekeeping plan for the country was ignored and the UN rights chief said crimes against humanity had probably been committed.
Syrian President Bashar al-Assad swiftly rejected the Arab League initiative for a joint mission with the United Nations to end the bloodshed, and shelling resumed in Baba Amr, a rebel bastion in Homs, rights monitors said on Monday.
The latest violence came as Navi Pillay, the UN High Commissioner for Human Rights, delivered a stark verdict on the consequences of the international community's failure to pass a UN resolution condemning the deadly crackdown.
"The nature and scale of abuses committed by Syrian forces indicate that crimes against humanity are likely to have been committed since March 2011," she told the UN General Assembly on Monday.
"The failure of the Security Council to agree on firm collective action appears to have emboldened the Syrian government to launch an all-out assault in an effort to crush dissent with overwhelming force."
The United States said it was considering whether a peacekeeping force would work while Russia, who with China vetoed a second UN resolution on Syria on February 4, said a ceasefire was needed before peacekeepers can be deployed.
The Syrian Observatory for Human Rights said Baba Amr was hit by artillery.
"The neighbourhood of Baba Amr has been subjected to sporadic shelling since 5am (1400 AEDT) by the Syrian army," the Britain-based group said in a statement.
Regime forces killed 18 civilians, among them four in Homs and two in nearby Rastan, including a 13-year-old girl. Eleven soldiers were also killed in clashes in various parts of the country.
Security forces also raided homes to arrest people in the southern Daraa province, cradle of the Arab Spring-inspired 11-month uprising against Assad's iron-fisted rule.
"There were fierce clashes between defectors and the army which stormed Lajat (also in Daraa province) and arrested the mothers of four dissidents," the Observatory said.
Despite the relentless violence, protests took place across Syria, where activists say more than 6000 people have died in the crackdown since last March.
Some denounced Assad and others supported the rebel Free Syrian Army, according to YouTube videos provided by the Local Coordination Committees, an activist network.
"Arab League!!! Thank you but we need more," said a placard students carried at a rally in Jabala, in Idlib province.
A government official said Syria was determined to crush dissent, regardless of the latest Arab initiative, the official SANA news agency reported.
"This decision will not prevent the Syrian government from fulfilling its responsibilities in protecting its citizens and restoring security and stability," the unidentified official was quoted as saying.
"Syria rejects decisions that are a flagrant interference in the country's internal affairs and a violation of its national sovereignty."
Activists say Assad's forces have killed at least 500 people in Homs since they began bombarding it on February 4 - the day Russia and China vetoed the second UN Security Council resolution.
That move prompted the pan-Arab bloc after marathon talks in Cairo at the weekend to ask for a joint Arab-UN peacekeeping mission, but the Syrian regime rejected the suggestion on Sunday.
The Arab League plan was on Monday welcomed by Britain, Germany, Italy and the European Union, while the White House said it was studying the idea.
US Secretary of State Hillary Clinton meanwhile hit out at the latest military action in Syria.
"It is deplorable that the regime has escalated violence in cities across the country, including using artillery and tank fire against innocent civilians," Clinton said in Washington at a joint press conference with Turkish Foreign Minister Ahmet Davutoglu.
Clinton said she welcomed a fresh chance to put international pressure on Syria when the 'Friends of Syria' group holds its first meeting in Tunisia next week and insisted more economic pressure would be exerted on Damascus.
"We will strengthen our targeted sanctions, bring the international community in condemnation of the actions of the Assad regime," the chief US diplomat said, alongside Davutoglu.
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