Syrian government forces intensified their shelling Wednesday of positions seized by rebels in the coastal province of Latakia, focusing on the Kasab border crossing into Turkey, an army officer said.
The province is the historic heartland of President Bashar al-Assad's clan and his Alawite sect of Shiite Islam, and territory the regime will not easily give up.
An AFP correspondent, who was with troops in the village of Al-Bassit, 15 kilometres (nine miles) west of Kasab, reported seeing columns of smoke rising over a mountain separating it from the border crossing.
"The army is relentlessly bombarding Kasab to stop the rebels from digging in" after capturing it on Sunday, the officer told AFP, adding that the nearby rebel-held village of Samra was also being targeted.
Omar al-Jeblawi, an activist on the ground, confirmed heavy shelling, and air strikes, near the border.
"There is a heavy bombardment of Kasab, using tanks and planes," Jeblawi told AFP by telephone.
And a rebel who identified himself only as Samer said the pounding was ferocious, but that there are "thousands of fighters ready to strike back against the army."
The officer said thousands of rebels had participated in a Friday assault on Kasab, in fighting that had killed 50 soldiers and left hundreds of rebels dead.
For its part, the Syrian Observatory for Human Rights said around 100 fighters had been killed on each side.
Signup to our newsletter and follow us on Facebook and Twitter!
"The regime mobilised thousands of soldiers and fighters from the National Defence Force (militia) to retake these areas," Observatory director Rami Abdel Rahman said.
"And there are many regime supporters, the majority of them Alawites, who have also volunteered to fight against the rebels.
"The regime did not expect this surprise offensive on its stronghold, but it will certainly not let the rebels keep control," he added.
On Tuesday, the rebels captured a hill known as Observatory 45, which strategically overlooks opposition-held areas of Latakia.
Separately on Sunday, Turkey downed a Syrian jet that Ankara said had violated its airspace, an act that Damascus described as "a flagrant act of aggression."
On Wednesday, Turkish Foreign Minister Ahmet Davutoglu said his country was ready to take any measures, including cross-border military operations, against threats to its national security.
"The Turkish republic is a powerful state and never hesitates to take any measures to protect its national security if need be," he told AFP in an exclusive interview.
Turkey backs the revolt that broke out against Assad in March 2011, and hosts more than 750,000 Syrian refugees.
But Syria has accused Turkey of giving cover to rebels in the assault on Kasab, and the officer in Al-Bassit said a Turkish tank had even fired at a Syrian army position.
More than 146,000 people have been killed in the three-year conflict and nearly half the Syrian population has been displaced.