Opposition activist Milad Fadl, contacted by AFP in Beirut, said the civilians were headed for Turkey to escape when regime forces opened fire.
In the evening, government troops launched an assault on the town of Binesh near the Turkish border. The shelling started at 21:30 pm (1930 GMT) with tanks deployed in Idlib city opening fire, an AFP reporter said.
Thousands began fleeing towards the nearby towns of Taftanaz, Al-Maara and Sardana, while shells hit many houses on the outskirts of Binesh and rebels took position to try to prevent the town from being taking over.
Earlier a rebel commander told AFP that Binesh "will become an inferno" if government troops launched an assault on the town east of Idlib.
"It will be like a volcano that has erupted that no-one can stop," said Abu Salmu, recalling how rebels and townspeople repulsed an attack four months ago after days of house-to-house combat that killed 16 of them.
The escalation came just hours after the UN Security Council adopted a statement urging Assad and the opposition to implement "fully and immediately" a peace plan by international envoy Kofi Annan.
Annan's plan calls for Assad to pull troops and heavy weapons out of protest cities, a daily two-hour humanitarian pause to hostilities, access to all areas affected by the fighting and a UN-supervised halt to all clashes.
Monitors say more than 9,100 people have been killed in the unrest that started with peaceful protests before turning into an armed revolt, faced with a brutal crackdown which has cost dozens of lives each day.
The Syrian National Council, the main opposition group, dismissed the UN statement, saying it offered "the regime the opportunity to push ahead with its repression in order to crush the revolt by the Syrian people."
Samir Nashar, an SNC executive committee member, told AFP by telephone from Istanbul the Council must "use its powers to stop these massacres."
SNC chief Burhan Ghalioun in Paris told AFP that the UN statement "has the merit of representing the common position of the international community against the policies of Bashar al-Assad. But it obviously does not meet the real needs of the Syrian people."
Turkish Foreign Minister Ahmet Davutoglu said the international community had to go further and develop "a joint plan of action."
"We continue to think that Syria is playing for time... In order for this human tragedy to end we must act together," he told reporters in Vienna. "Just making calls is not enough."
He added: "A regime fighting against its own people, trying to keep the status quo, cannot survive."
Before the evening offensive, 62 people were killed across the country on Thursday, including 35 civilians, the Observatory said, adding that 18 soldiers and nine army deserters also died in fierce clashes.
The reports could not be confirmed due to restrictions on the movements of foreign media.
On the rebel side, the Free Syrian Army said it had set up a military council to coordinate hit-and-run strikes around the capital, so far largely spared the worst violence.
The official media in Damascus Thursday played up the lack of any threat or ultimatum in the non-binding Security Council statement.
After intense negotiations between major UN powers, Russia and China signed up to the Western-drafted text which also calls on Assad to work toward a democratic transition.
Russia and China have vetoed two Security Council resolutions on Syria, arguing they were unbalanced and aimed at regime change.
The Security Council awaited a formal response from Syria to its call.
With a veiled warning of future action, it called on Assad and the opposition to work "towards a peaceful settlement of the Syrian crisis and to implement fully and immediately (Annan's) initial six-point proposal."
US Secretary of State Hillary Clinton praised the UN statement and warned Assad to carry out the plan or "face increasing pressure and isolation."
European countries still want to press for a full, binding resolution, with French envoy Gerard Araud calling the statement "a small step by the Security Council in the right direction."
Meanwhile, State Department spokeswoman Victoria Nuland said Clinton will attend the next "Friends of Syria" talks in Istanbul on April 1.
Nuland said the meeting will build on previous efforts to end the violence, enabling the delivery of humanitarian aid and launching a political process aimed at replacing Assad.