BEIRUT: 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 in the U.S. between President Barack Obama and his Republican challenger, Mitt Romney,

Both men said they would not send American troops to Syria, but Romney pledged to help arm rebels after vetting the intended recipients, earning him praise from Syrian opposition leaders. Obama warned of the risk of giving the rebels heavy weapons that could fall into 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 pounded rebel positions with warplanes and helicopter gunships.

On Tuesday, government 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. The activist group reported more than two dozen casualties among the rebels, but did not have the breakdown of killed and wounded.

Opposition fighters seized Maaret al-Numan, which lies along the main highway between Aleppo and the capital Damascus, earlier this month. Their presence has disrupted the regime's ability to send supplies and reinforcements to the northwest. This has hampered the government's fight in Aleppo, where 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, apparently at 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.

An international human rights group, meanwhile, said the Syrian air force stepped up attacks with cluster bombs in the past two weeks, dropping more over a wider area, despite Damascus' denials that it has used the widely banned munitions.

Cluster bombs open in flight, scattering smaller bomblets over a wide area. Many of the bomblets don't explode immediately, posing a threat to civilians long afterward.

The New York-based Human Rights Watch, citing amateur videos and interviews with victims, residents and activists, said it has received new information about more than 35 sites where cluster bombs were dropped.

The violence in Syria has killed more than 34,000 people, 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.

Abdelbaset Sieda, the head of the Syrian National Council, 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.