Syrian rebels shot down a helicopter on Wednesday as troops tried to retake a key town, a watchdog said, and peace envoy Lakhdar Brahimi warned that the conflict risks setting the region ablaze.
The fierce battle for the Damascus-Aleppo highway raged around the northwestern town of Maaret al-Numan even as Brahimi appeared to have won tentative support for a ceasefire.
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."
He 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.
"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, also asking for international support.
The conflict 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.
At times it has spilled over into neighbouring nations. Turkey-Syria 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.
Despite the renewed cross-border fire, Foreign Minister Ahmet Davutoglu said Ankara backs Brahimi's ceasefire proposal.
"In principle, we consider a ceasefire... to be declared during the Eid al-Adha as useful," Davutoglu told television station A Haber.
Davutoglu also said the plan was 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.
Brahimi spokesman Ahmad Fawzi told AFP 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.
Damascus says it is ready to discuss the truce proposal with Brahimi while the exiled opposition says it would welcome any ceasefire but insists the ball is in the government's court.
"The Syrian side is interested in exploring this option and we are looking forward to talking to Mr Brahimi," foreign ministry spokesman Jihad Maqdisi told AFP.
The opposition Syrian National Council said it expected the rebel Free Syrian Army to reciprocate any halt to the violence, but the government had to act first.
"We would welcome any halt to the killings but we think the appeal needs to be addressed first to the Syrian regime, which has not stopped bombarding Syrian towns and villages," SNC leader Abdel Basset Sayda told AFP.
On the battlefront, 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, the area's largest.
Violence killed at least 108 people nationwide on Wednesday, including 33 civilians, said the Observatory.
Some 33,000 people have been killed in the uprising, among them 2,300 children, the Observatory says.