At least 40 people were reportedly killed in the latest violence a day before Major General Robert Mood briefs the United Nations Security Council following the suspension of his UN observer mission in Syria.

On the diplomatic front, US President Barack Obama is due to meet his Russian counterpart Vladimir Putin later Monday at a G20 summit to discuss differences over what to do about the bloody conflict.

And France said that, after Mood's briefing, it and other members of the Security Council would examine steps to take in the wake of the suspension of the United Nations Supervision Mission in Syria, or UNSMIS.

Home to several rebel strongholds, the central city of Homs has been under intermittent attack by the regime ever since Baba Amr district was pounded relentlessly for a month before being retaken by government forces in March.

"Shelling and shooting renewed in Homs city, with explosions heard in the Khaldiyeh neighbourhood," said the Syrian Observatory for Human Rights, which put Monday's death toll across the country at 40 people.

In the southern province of Daraa, considered the cradle of a revolt against President Bashar al-Assad's rule that erupted in March 2011, the town of Tafas was under siege and shelling by regime troops, according to the Free Syrian Army.

"The regular army was able to break into the town from the southern entrance, where they launched a campaign of raids, but did not penetrate the whole town," said FSA spokesman Louay Rashdan.

"The FSA fighters are still resisting," Rashdan said, noting that young men from Daraa and Dael came to aid the Tafas fighters in fierce clashes with regime troops.

UN rights chief Navi Pillay on Monday demanded the immediate cessation of such bombardments of populated areas, warning that such violence amounted to crimes against humanity.

"The government of Syria should immediately cease the use of heavy armaments and shelling of populated areas, as such actions amount to crimes against humanity and possible war crimes," she told the Human Rights Council in Geneva.

Pillay also called for a probe into an attack on a UN convoy a week ago in the town of Al-Haffe in the northwestern province of Latakia.

She said those responsible for such attacks on UN observers must be brought to justice.

The Observatory said on Monday that seven people, including two rebel commanders, were killed in an explosion in Mohassan town in the eastern province of Deir Ezzor.

It said clashes and shelling continued through the night in several areas of Damascus province, including the towns of Douma and Qudsaya which have been under regime bombardment for the past five days.

Speaking to AFP via Skype, an anti-regime activist who identified himself as Mahmoud Doumani said that there was much "destruction in Douma" and that the "situation is very sad."

"The army has escalated its operations in Douma in recent days," Doumani said, adding that families who have not fled the town are in hiding.

"Regime forces have destroyed homes, farms and many mosques over the past week. But in the past couple of days, in the absence of the UN monitors, attacks on Douma have become even worse than before."

The UN observers were severely restricted in their movements, Doumani said, "but at least there was some kind of plan. Now the Syrian army is operating without any obstacles whatsoever. This is very dangerous for us."

UNSMIS on Saturday suspended its operations two months into its three-month mandate, blaming the intensifying violence threatening its 300-strong force.

And on Tuesday, General Mood urged the warring parties to "allow women, children, the elderly and the injured to leave conflict zones, without any preconditions and ensure their safety."

Main opposition group the Syrian National Council on Sunday urged the United Nations to pressure the Syrian regime using Chapter VII of the UN charter, which allows measures to be imposed on a country under penalty of sanctions or force.

The opposition specifically demanded that observers tasked with monitoring a UN-backed ceasefire – that has been flouted daily since going into effect on April 12 – be armed.

Meanwhile, Russia said it is preparing to send two amphibious assault ships to the Syrian port of Tartus where Moscow has a strategic naval base to ensure the safety of its citizens, Interfax news agency reported.