The main opposition bloc in Syria has accused Lebanon's Hezbollah movement of fighting alongside regime forces in Damascus.
Hezbollah fighters crossed into Homs province, in central Syria, on Saturday and attacked three villages in the Qusayr region near the Lebanese border, the rebel Syrian National Council said.
The operation resulted in ''civilian casualties and the exodus of hundreds of people'', the opposition said.
The council said Hezbollah was using heavy weapons ''under the auspices of the Syrian regime army'', and that Beirut was responsible for ending this ''aggression''.
In Lebanon, a Hezbollah official said three Lebanese Shiites were killed in clashes in Syria while 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 Bashar al-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 Dr Assad's regime and the Sunni-led March 14 opposition movement and its allies supporting the rebellion.
The British-based Syrian Observatory for Human Rights said the three slain Lebanese were members of pro-regime militias who had been trained by Hezbollah, and it reported that at least 87 people were killed across Syria on Sunday, including 35 rebels.
Rebels have pressed the ''battle of the airports'' in northern Syria, striking three key facilities.
In Aleppo, fierce clashes raged between Syrian troops and several rebel battalions, with insurgents seizing an army checkpoint near Nayrab military airbase, according to the Observatory.
Nayrab is next to Aleppo international airport, a key target for the rebels, who have also been battling troops guarding Kwiriss military airbase east of the city and Mannagh airbase to the north.
The Observatory said the rebels lost five men in the eastern province of Deir al-Zor before taking control of the Kibar checkpoint, seizing weapons and ammunition and killing at least four soldiers.
United Nations rights chief Navi Pillay urged that some sort of international action be taken against Dr Assad ''immediately'' and for the president to be investigated for ''crimes against humanity and war crimes''.
''If there is doubt or hesitation it is because people are assessing the value of military intervention in places like Libya, Syria and Afghanistan,'' Ms Pillay told Britain's Channel 4 television.
The Syrian conflict, which began in March 2011 with unprecedented mass protests and became an armed insurgency, has left nearly 70,000 people dead, according to the United Nations.
UN-Arab League peace envoy Lakhdar Brahimi on Sunday said the world community should back a Syrian opposition offer to begin talks with the regime.
