A commander of the rebel Free Syrian Army warned on Wednesday that his forces would target Lebanon's Hezbollah unless the powerful militant group stops shelling territory held by the insurgents.
The ultimatum, which expires on Thursday, came after the outgunned rebel fighters shot down a regime warplane following a deadly air strike that killed 20 people in a Damascus province town.
General Selim Idriss, the FSA chief of staff, told AFP that Hezbollah had long been taking part in hostilities in Syria, but had gone too far by shelling villages near Qusayr in Homs province from the Bekaa valley in Lebanon.
"Hezbollah is abusing Lebanese sovereignty to shell Syrian territory and Free Syrian Army positions," said Idriss.
"In the past week... Hezbollah has been shelling into villages around Qusayr from Lebanese territory, and that we cannot accept."
The commander said the rebels were giving Hezbollah a 48-hour deadline to stop the attacks and "as soon as the ultimatum ends, we will start responding to the sources of fire".
Rebels in the Qusayr area would would be backed by FSA fighters "equipped with long-range weapons from other areas," he said.
The FSA had also asked the Lebanese president and premier to intervene, said Idriss, but the office of Prime Minister Najib Mikati denied any contact with the Syrian rebels.
Hezbollah has repeatedly denied sending fighters into Syria, though its leader Hassan Nasrallah acknowledged in October 2012 that party members had fought Syrian rebels but said they were acting as individuals and not under the group's direction.
Lebanon is sharply divided over the Syrian conflict, with the Sunni-led March 14 movement supporting the revolt and the Shiite Hezbollah and its allies backing the regime of President Bashar al-Assad.
On Wednesday, a Lebanese military judge demanded the death penalty for former minister Michel Samaha and Syrian security chief Ali Mamlouk, who face charges of plotting attacks on political and religious figures in Lebanon.
Signup to our newsletter and follow us on Facebook and Twitter!
Samaha, a Christian politician with close links to the Damascus regime, was accused of stoking sectarian strife in his country.
On the ground in Syria, rebels managed to shoot down a warplane in Hammuriyeh, after an air strike on the central town killed 20 people, including a woman and child, a watchdog and activists said.
Amateur video shot by activists and distributed by the Syrian Observatory for Human Rights showed a warplane firing from the sky and then going down in flames after apparently being hit.
"It's gone up in flames... Free Syrian Army air defence battalions have hit a MiG warplane!" cried the cameraman.
Lacking sophisticated weaponry, rebels have frequently used heavy machineguns to shoot down warplanes deployed to strike insurgent enclaves across the country.
An unverified video filmed by activists in Hammuriyeh and distributed on Facebook showed residents in the aftermath of the strike pouring water on a burning corpse on the ground, and another two in destroyed vehicles.
In Damascus, a footballer was killed and four others wounded when two mortars smashed into a stadium in the capital, a sports official told AFP. State news agency SANA blamed "terrorists" for the attack.
The day before, mortar rounds fired by the rebels had exploded near the southern wall of Tishreen presidential palace, one three official residencies in the capital.
It was the first such attack on a presidential palace reported by state media.
Elsewhere, clashes raged in the northern province of Aleppo around three key airports, days after rebels launched an assault to seize the international airport and other air bases in the region.
At least 122 people were killed nationwide on Wednesday, among them 65 civilians, said the Observatory, which relies on a network of sources for its information.
The United Nations says at least 70,000 people have been killed in the near two-year conflict, and puts the number of Syrians who have fled their homeland at more than 850,000.