Syrian security forces launched a blistering offensive Saturday on the northern region of Idlib, sending people fleeing for their lives, while in Damascus, UN-Arab League envoy Kofi Annan expressed "grave concern" after meetings with Syrian President Bashar Assad.

Thick black smoke billowed into the sky over Idlib as armed rebels tried to fend off the attack. Armed opposition fighters were seen running through the streets, taking cover behind the corners of buildings, in clashes with the troops.

Reports of this latest military operation raised fears that Assad's regime was planning a new all-out offensive in Idlib similar to last month's bloody siege farther south in Homs. Security forces killed at least 72 people in Homs on Friday alone, according to activists.

Wounded fighters, meanwhile, were pulled into trucks to be taken to clinics for treatment. Troops moving to encircle the town battered it with dozens of tanks shells, The Associated Press reported.

Troops, tanks move in

Military reinforcements poured into Idlib this week, including dozens of tanks and armoured personnel carriers, said activists, who reported that dozens have been killed in the area in recent days.

"What's happening in Syria is unbelievable," Osama Monajed of the opposition Syrian National Council, told CBC News on Saturday. Assad's regime is "at war against its own people," he said.

Annan's meetings this weekend with Assad in Damascus are part of international efforts to halt the fighting and arrange talks between the two sides to end the Syrian conflict which, according to the latest UN estimate, has killed more than 7,500 people since it began a year ago.

Annan, the former UN secretary general, called the talks "candid and comprehensive" but also expressed "grave concern at the situation" and urged Syria's government to take concrete steps to end the crisis, a UN spokesman said.

Annan put several proposals on the table regarding stopping the violence, providing humanitarian agencies and the International Red Cross with access, releasing detainees and starting a "political dialogue" in Syria.

Assad, meanwhile, said any political dialogue was doomed to fail "as long as there are armed terrorist groups that work to spread anarchy and destabilize the country," according to the state news agency SANA.

Syria's government blames the uprising on terrorists acting out a foreign conspiracy, not protesters seeking change.

Rebels have so far rejected his call to join the talks.

Aim is peaceful resolution, Russia says

Meanwhile, Russian Foreign Minister Sergey Lavrov on Saturday told an Arab League meeting his government is not "protecting any regimes" but merely trying to "protect international law" and "promote a peaceful resolution" to Syria's crisis.

Russia is seen as the Syrian government's top ally and Arab leaders are angry over Moscow's blocking of international pressure on Assad to step down.

Lavrov said the immediate mission now is to halt violence and enable humanitarian assistance to reach those in need in Syria.

U.S. Secretary of State Hillary Clinton is to meet Lavrov in New York on Monday. Clinton said she's hoping Russia will play a "constructive role" in working to end the bloodshed in Syria.