Even as Brahimi appeared to be winning support within Syria for a ceasefire, rebels shot down an army helicopter on Wednesday while a fierce battle for the Damascus-Aleppo highway raged around the north-western town of Maaret al-Numan.
The UN and Arab League envoy warned of the conflict spreading as he visited neighbouring Lebanon, the latest leg of a Middle East tour aimed at ending more than 19 months of bloodshed.
"This crisis cannot remain confined within Syrian territory," the veteran trouble-shooter told reporters.
"Either it is solved, or it gets worse … and sets [the region] ablaze."
His words came just hours before reports of clashes across the restive Syria-Lebanon border.
A Lebanese security official said armed men in Lebanon used machineguns to shoot into Syrian territory and the Syrian army responded with rounds fired from tanks and machineguns.
"The Syrian army fired shells into Lebanon after unidentified armed men opened fire across the border near the village of Aboudiyeh" in northern Lebanon the official said, adding there were no casualties.
Ever since the outbreak of the anti-regime revolt in Syria, multiple exchanges of fire have taken place across the border.
Lebanon has made two official complaints against the Syrian authorities over territorial violations, while the regime of President Bashar al-Assad accuses Lebanon of allowing arms and fighters to enter into Syria illegally.
The conflict has at times also spilled over into neighbouring Turkey. Bilateral tensions have soared, with Ankara taking an increasingly strident line since a shell fired from inside Syria killed five Turks on October 3.
A mortar bomb fired from Syria struck Turkish territory on Wednesday but caused no casualties, Turkish NTV reported.
Turkey struck back with retaliatory fire, as it has systematically done since the first incident, Anatolia news agency said.
Brahimi said a truce for the four-day Muslim Eid al-Adha holiday from October 26 would be "a microscopic step on the road to solving the Syria crisis".
"The Syrian people, on both sides, are burying some 100 people a day," said Brahimi.
Ready to discuss truce
"Can we not ask that this toll falls for this holiday? This will not be a happy holiday for the Syrians, but we should at least strive to make it less sad.
"If the Syrian government accepts, and I understand there is hope, and if the opposition accepts," a truce would be a step "towards a more global ceasefire".
Arab League chief Nabil al-Arabi, who met Brahimi on Tuesday, backed the call for an Eid truce and Turkey's Foreign Minister Ahmet Davutoglu said Ankara supports Brahimi's proposal.
"In principle, we consider a ceasefire … to be declared during the Eid al-Adha as useful," Davutoglu told television station A Haber.
Davutoglu said the plan was also backed by major Syrian ally Iran, adding that Turkish Prime Minister Recep Tayyip Erdogan and Iranian President Mahmoud Ahmadinejad had discussed it at a regional summit in Azerbaijan.
Damascus says it is ready to discuss the truce with Brahimi, while the opposition Syrian National Council said it expects the rebel Free Syrian Army to reciprocate any halt to the violence, but that the government must act first.
Brahimi spokesperson Ahmad Fawzi said the envoy would "soon go to Damascus." His tour has already taken him to Egypt, Saudi Arabia and Turkey, predominantly Sunni Muslim countries, as well as Shiite-majority Iran and Iraq.
Halting regime efforts
The conflict between Syrian troops and rebels began in March 2011 with pro-reform protests inspired by the Arab Spring, but is now a civil war pitting mainly Sunni rebels against President Bashar al-Assad's regime dominated by his minority Alawite sect, an offshoot of Shiite Islam.
On the battlefront on Wednesday, rebels shot down a helicopter gunship as the army fought to recapture Maaret al-Numan, said the Syrian Observatory for Human Rights.
Amateur video posted on YouTube showed a helicopter spiralling downwards and exploding, as onlookers cried: "Allahu akbar [God is greatest]!"
Warplanes targeted the rebel blockade of the highway that has halted regime efforts to reinforce Aleppo, a theatre of intense fighting for three months.
The early morning raids targeted Maaret al-Numan and nearby villages that fell to the rebels a week ago as they pushed to create a "buffer zone" abutting Turkey, the Observatory said.
However, analysts say regime air supremacy is no longer decisive, as multiple fronts are stretching its capabilities.
The fighting flared as rebels attacked a convoy of tanks in the town of Maarhtat as it headed for Wadi Deif army base.
Violence killed at least 108 people nationwide on Wednesday, including 33 civilians, said the Observatory which calculates some 33 000 people have died in the uprising, among them 2 300 children. – Sapa-AFP