Syrian warplanes on Tuesday struck a strategic rebel-held town in the country's north in an attempt to reopen a key supply route, activists said, as a U.N.-proposed cease-fire meant to start this week appeared increasingly unlikely to take hold.

The U.N.-Arab League envoy to Syria has suggested that both sides in Syria's 19-month-old conflict lay down their arms during Eid al-Adha, a four-day Muslim holiday that begins Friday. However, neither Syrian President Bashar Assad nor rebels fighting to topple him have committed to a truce, and international envoy Lakhdar Brahimi has not said how such a truce would be monitored.

Syria's stalemated civil war, which has frequently spilled over Syria's borders and threatens to destabilize an already volatile region, featured prominently in the final pre-election debate Monday between President Barack Obama and his Republican challenger, Mitt Romney,

Both men said they would not send U.S. troops to Syria, but Romney pledged to help arm Syria's rebels after vetting the intended recipients, a stance that earned him praise from Syrian opposition leaders. Obama warned of the risk of giving the rebels heavy weapons that could fall in the wrong hands and later be used against the U.S. or its allies.

"By not arming the (rebel) Free Syrian Army with heavy weapons, he (Obama) is giving Assad the upper hand," said Muhieddine Lathkani, a member of the Syrian National Council, an umbrella of opposition groups.

The rebels have said they need heavier weapons to counter Assad's military superiority, particular from the air. Since the summer, the regime has increasingly bombed rebel positions with warplanes and combat helicopters.

In some of the heaviest fighting Tuesday, Syrian aircraft attacked Maaret al-Numan and the village of Mar Shamsheh, as troops and rebels battled over a nearby Syrian military camp that has been under siege for days, according to the Britain-based Syrian Observatory for Human Rights, an activist group.

Earlier this month, opposition fighters seized Maaret al-Numan, which lies along the main highway between Aleppo and the capital Damascus, and their presence there has disrupted the regime's ability to send supplies and reinforcements to the northwest. That has hampered the government's fight in Aleppo, where government troops are bogged down in a bloody fight for control of the country's largest city.

Amateur video posted Monday showed rebel fighters unleashing heavy bursts of machine-gun fire toward a convoy heading to the besieged army camp. Syria restricts access to foreign reporters and the authenticity of the video could not be independently confirmed.

Also Tuesday, an anti-aircraft shell from Syria hit an empty room of a health center some 200 meters inside Turkey, causing no casualites, the private Dogan news agency reported. Assad's forces and rebels have been fighting in the area but on the Syrian side of the border.

Turkey has been retaliating for stray Syrian artillery fire a shell slammed into a Turkish border town on Oct. 3, killing five civilians.

The violence in Syria has killed more than 34,000 people in the past 19 months, said Observatory chief Rami Abdul-Rahman, whose group relies on a network of activists on the ground. This figure includes civilians and rebel fighters, but also more than 8,000 regime soldiers, he said.

Abdul-Rahman said there were no signs on inside Syria that rebels or government troops were preparing to halt fighting during Eid al-Adha, or the Feast of Sacrifice. "Until we see evidence on the ground, it means it (the cease-fire) is a dream only," he said.

The head of the Syrian National Council said chances for a truce are slim.

SNC chief Abdelbaset Sieda told The Associated Press on Tuesday that rebel fighters are willing to halt fighting during the holiday, but will respond if attacked. He said he doubts the regime will honor the cease-fire and that Brahimi's plan is too vague.

"Brahimi hasn't any mechanism to observe the situation," Sieda said by phone from Stockholm, Sweden. "Now he is saying every side can do that (halt fighting) by itself."

In Damascus, Syria's deputy foreign minister, Faisal Mekdad, said the Assad regime is cooperating with Brahimi. "We are always optimistic," he said when asked about the chances of a cease-fire.

However, Brahimi said after a visit to Damascus on Sunday that he did not get a commitment to the truce from the regime.

The relentless fighting in Syria has forced hundreds of thousands of people to flee the country to escape the violence.

The U.N. refugee agency said Tuesday that Lebanon has become the third Syrian neighbor to host more than 100,000 refugees from Syria's civil war.

At least 101,283 people have registered as refugees in Lebanon, said agency spokeswoman Melissa Fleming. This compares to more than 105,000 in Jordan, at least 101,000 in Turkey, more than 42,000 in Iraq and 6,800 in North Africa.

Governments bordering Syria estimate tens of thousands more Syrian refugees have not yet registered, including an estimated 70,000 in Turkey who live outside refugee camps, she said.

Jordan alone says it hosting some 210,000 Syrian refugees, which Amman says has strained the country's health care, water and electricity sectors.