Exiled opposition figure Moaz Alkhatib visited Syria on Sunday for the first time since fleeing last year, as rebels said President Bashar al-Assad's forces embarked on counter-offensives in various parts of the country.
In the central city of Homs, heavy fighting broke out between loyalist forces and opposition brigades dug in preparation for an onslaught, opposition sources said.
Opposition campaigners said the counter-offensive appeared to be part of a new strategy by Assad focusing on regaining three rebel-held regions that pose a threat to his grip on Damascus and supply lines from coastal regions, where a large proportion of his minority Alawite sect live.
"We are probably seeing the first stage of a major onslaught on Homs," said Mohammad Mroueh, a member of an opposition 'crisis committee'.
Alkhatib, president of the Syrian National Coalition, a group of anti-Assad interests that has sought international recognition, crossed into northern Syria from neighboring Turkey and toured the towns of Jarablus and Minbij.
Alkhatib, a 52-year-old former preacher at the Umayyad Mosque in Damascus, was chosen in November to head the SNC and his visit appeared aimed at overcoming skepticism among some of the disparate rebel forces towards his Cairo-based Coalition.
He has said he is ready for talks with representatives of Assad's government to seek a political solution to a conflict which erupted nearly two years ago and has descended into a civil war in which around 70,000 people have been killed.
The SNC says any talks must focus on Assad's departure while rebel leaders insist he depart before talks can start.
Before entering Syria, Alkhatib attended a meeting of 220 rebel commanders and opposition campaigners in the Turkish city of Gaziantep to elect an administration for Aleppo province, home to 6 million people.
Assad, in an interview with British newspaper The Sunday Times, said his government was prepared to talk to fighters who lay down their weapons but insisted he would not leave the country or step aside under foreign pressure.
"We are ready to negotiate with anyone, including militants who surrender their arms," he said according to a transcript released by state media. However there would be no talks with "terrorists who are determined to carry weapons."
But Assad, who inherited power from his father in 2000, said he was not going anywhere. "No patriotic person will think about living outside his country. I am like any other patriotic Syrian," he told the newspaper.
In Aleppo, home to one of Syria's two oil refineries and on a road linking coastal army supply bases to Damascus, rebels were fighting off an incursion by a pro-Assad militia known as shabbiha, opposition sources said.
They said opposition fighters captured a police academy on the outskirt of Aleppo, after days of fighting in which rebels killed 150 soldiers, while sustaining heavy casualties.
Further east, Iraqi military sources said Iraq shut a border crossing with Syria after rebels seized the Syrian side of the frontier post close to the Syrian town of Yaarabiya.
"Iraqi authorities were ordered to shut off Rabia border crossing until further notice because of the Syrian government's lack of control over the other side of the post," police said.
In Amman, Jordan's national carrier Royal Jordanian said it had stopped flying over Syrian air space for security reasons. The airline stopped its regular flights to Damascus last year along with some other carriers.