Fierce battles raged Wednesday in Syria's coastal Latakia province near the Turkish border, days after rebels seized a frontier crossing, said the Syrian Observatory for Human Rights.
On Tuesday, the rebels captured a hill known as Observatory 45, which strategically overlooks opposition-held areas of Latakia, the historic heartland of President Bashar al-Assad's clan and his Alawite sect of Shiite Islam.
The latest fighting raged around Kasab, home to the border crossing seized at the weekend, as well as Observatory 45 and Samra, also captured by rebels this week.
According to Omar al-Jeblawi, an activist on the ground, the army was retaliating with heavy shelling including air strikes near the Turkish border.
"There is heavy shelling on Kasab, using tanks and planes," Jeblawi told AFP by phone.
On Sunday, Turkey downed a Syrian jet which Ankara said had violated its airspace. Damascus described the downing as "a flagrant act of aggression".
Turkey backs the revolt which broke out against Assad in March 2011, and hosts more than 750,000 Syrian refugees.
The Observatory and opposition sources said the army has deployed a large number of fighters to battle the rebels, including pro-regime National Defence Force militiamen.a
A rebel fighting the army said the clashes in flashpoint areas of Latakia were fierce.
"There are thousands of (opposition) fighters ready to strike back against the army," said the rebel, who identified himself only as Samer.
Nearly half of Syria's population has been displaced by the three-year war and more than 146,000 people have been killed.