Syrian government forces pressed on with deadly assaults on Friday, killing around 50 civilians, monitors said, on the eve of a peace mission by international envoy Kofi Annan.
And in a new blow to the regime after this week's resignation of a deputy cabinet minister, a dozen army officers defected, including six generals and a woman lieutenant, going across the border to Turkey, reports said.
On the diplomatic front Russia, one of Syria's last remaining allies along with China, criticised as "unbalanced" a new US-led initiative to push through a damning UN Security Council resolution.
Underscoring divisions among world powers, US State Department spokeswoman Vitoria Nuland said Washington is "not overly optimistic" about the passing of the resolution "in the near future."
Russian Foreign Minister Sergei Lavrov is to explain Moscow's stance in talks on Saturday in Cairo with his Arab counterparts.
But first he will meet Annan in the Egyptian capital late on Friday before the UN-Arab League envoy heads to Syria, a UN spokesman in New York said.
Regime troops stormed a village in Idlib, and attacked other districts there, reflecting growing fears that the northwestern province will meet the same fate as the battered rebel stronghold of Baba Amr in the city of Homs.
"Troops attacked the village of Ain Larose and opened fire killing 13 civilians," said Rami Abdel Rahman, head of the Britain-based Syrian Observatory of Human Rights told AFP in Beirut.
They were among nearly 50 people killed in the assaults in Idlib and elsewhere across the country by regime forces, including the rebel province of Homs where rocket and mortar attacks claimed 10 lives.
The deaths came as tens of thousands of people demonstrated against the regime across the country, with huge demonstrations taking place in the second city Aleppo.
Protesters demanding the ouster of President Bashar al-Assad chanted: "Assad your days are numbered" and "May God damn your soul."
They also called for the arming of the rebel Free Syrian Army (FSA).
But Annan who is due to meet Assad in Damascus on Saturday morning, on the heels of a two-day visit by UN humanitarian chief Valerie Amos, has warned against further militarisation of the crisis, in remarks echoed by EU foreign ministers, as well as by Washington.
UN chief Ban Ki-moon said Annan would not leave Damascus, and would only talk with opposition leaders outside Syria after he leaves on Sunday.
Following his departure, he will visit other countries in the region, Ban told reporters in New York, without elaborating, though diplomatic sources in Ankara said Annan's first stop from Damascus would be Turkey.
Ban stressed that Annan's top priority must be to secure an immediate ceasefire between government and opposition forces, and then "to urge Assad to facilitate humanitarian assistance and access."
Faced with a groundswell of pressure to end the bloodshed, Syria said it was ready to allow the United Nations to conduct a humanitarian mission.
Signup to our newsletter and follow us on Facebook and Twitter!
Amos said in Ankara on Friday that a "joint preliminary humanitarian assessment mission" had been agreed, to provide assistance to people urgently in need of it.
The mission would only be a "first step", she said, insisting that Damascus must allow aid groups "unhindered access to evacuate the wounded and deliver desperately needed supplies."
No UN aid agencies are currently allowed into Syria, where the opposition says 8,500 have been killed in the year-long conflict, and information is scarce on the details of the civilians' needs.
But a UN spokeswoman in Geneva said 1.5 million people might be in need of food aid.
Following Amos's visit, China on Friday offered $2 million of humanitarian aid to improve the humanitarian conditions in different parts of Syria.
Regime forces have been massing troops around Idlib for days to root out rebel FSA fighters.
Armoured units have surrounded the hilly district of Jabal al-Zawiya, where rebel fighters have been active, and there were reports that civilians were fleeing en masse.
Abdel Rahman said the army was hunting down rebels in the area because "the largest number of deserters are in Jabal al-Zawiya."
Activists fear that Idlib could suffer the same fate as the Baba Amr, which was stormed by government troops on March 1 after a month of shelling.
The UN humanitarian chief briefly toured Baba Amr on Wednesday with a Syrian Red Crescent team, and said the district has been "totally destroyed."
"There were hardly any people left there," Amos said.
Ahead of his visit, Annan urged "the Syrian opposition to come together to work with us to find a solution that will respect the aspirations of the Syrian people."
Speaking in Cairo earlier this week, the former UN chief argued that the further militarising the conflict would only make the situation worse.
"I hope that no one is very seriously thinking of using force in this situation," he said.
On Friday, the opposition and Turkish news agency Anatolia reported that a dozen Syrian army officers had defected and fled to Turkey where they were to join rebels based there.
Anatolia said four generals and two colonels who had been stationed in Damascus, Homs and Latakia, had crossed the border into Turkey's southern province of Hatay to join the Syrian rebels.
Fahd al-Masri, an advisor for Syrian opposition group the Higher Revolutionary Council, told AFP that six brigadier generals had defected in the past 48 hours along with four colonels, a lieutenant colonel, a major and a female lieutenant.