Rebels pressed the "Battle of the airports" in the north on Sunday, attacking three key facilities as the main opposition bloc accused staunch Damascus ally Hezbollah of "military intervention" in Syria.
Peace envoy Lakhdar Brahimi, meanwhile, urged backing for an opposition offer for talks with the regime of Bashar al-Assad after the UN rights chief called for international action against the Syrian president.
On the ground, troops parried fierce rebel attacks near Aleppo airport, the adjacent Nayrab military airbase and Kwiyres airbase east of Aleppo city overnight, the Syrian Observatory for Human Rights said.
The insurgents launched their offensive against airports in the north on February 12, and have since seized Al-Jarrah military airport and Base 80, which was tasked with securing Aleppo's civilian airport.
The opposition Syrian National Council accused members of Lebanon's powerful armed movement Hezbollah of entering the nearly two-year conflict in Syria.
It said Hezbollah fighters crossed into Homs province on Saturday and attacked "three Syrian villages in the Qusayr region near the Lebanese border".
The operation led not only to "civilian casualties and the exodus of hundreds of people," but has also "stoked sectarian tensions" in the area, the SNC said.
The SNC said Hezbollah was using heavy weapons "under the auspices of the Syrian regime army".
This is a "serious threat to Syrian-Lebanese relations and regional peace and security," it said, adding that Beirut has a responsibility to end this "aggression."
In Lebanon, a Hezbollah official said on Sunday that three Lebanese Shiites were killed in clashes in Syria when acting in "self-defence", without specifying if they were Hezbollah members.
Lebanon's Hezbollah is Shiite, while most of Syria's population and the rebels battling Assad's regime are Sunni. The ruling clan and many of its most fervent supporters belong to the minority Alawite sect, an offshoot of Shiite Islam.
Lebanon is sharply divided over the uprising in its larger neighbour, with Hezbollah and its allies in the ruling coalition backing Assad's regime and the Sunni-led March 14 movement and its allies supporting the rebellion.
The Observatory reported that eight foreign fighters in Aleppo were among 36 rebels killed in Syria on Saturday, along with 37 civilians and 31 government troops.
UN rights chief Navi Pillay urged that some sort of international action be taken against Assad and he be investigated for "crimes against humanity and war crimes."
"We urge that action be taken immediately. If there is doubt or hesitation it is because people are assessing the value of military intervention in places like Libya, Syria and Afghanistan," Pillay told Britain's Channel 4 television.
The Syrian conflict, which began in March 2011 with unprecedented mass protests and morphed into an armed insurgency after a harsh state crackdown, has left nearly 70,000 people dead, according to the United Nations.
UN-Arab League peace envoy Brahimi on Sunday said the world community should back a Syrian opposition offer to begin talks with the regime and he proposed they be held in United Nations offices.
Brahimi said opposition leader Ahmed Moaz al-Khatib's offer to negotiate with some regime figures "opened the door and challenged the Syrian regime to confirm what it constantly says about being prepared to hold dialogue."
Israeli premier Benjamin Netanyahu, meanwhile, said his country will maintain close tabs on its border with Syria and only let people to cross in "exceptional circumstances" -- a day after seven injured Syrians were allowed in Israel.
On Saturday, a military source told public radio Israel had prepared designated areas along the frontier to receive Syrian refugees under the auspices of the United Nations.