BEIRUT — A battle raging for control of a border town critical to the supply of men and arms to opponents of the regime of Syrian President Bashar Assad may determine whether the two-year rebellion will survive, say analysts and a rebel officer.

Rebels have held the town of Qusayr a few miles from Lebanon for over a year, but fighters for Iran-backed Hezbollah, a U.S.-designated terror group, have added renewed strength to the Syrian government offensive to crush the rebel haven.

The shift comes as Secretary of State John Kerry met with Arab and European allies Wednesday on stepping up support for Syria's rebels to help them "fight for the freedom of their country" if Assad refuses to join hoped-for peace talks in Geneva next month.

Rebels were speeding about Thursday in pickups and cars, some of them mounted with anti-aircraft guns, to fend off the offensive that some reports say has taken over much of the town. Opposition fighters appeared to be under increasing strain as Assad's tanks and artillery pounded the town, and warplanes bombed it from the sky, according to reports provided to the Associated Press.

If the regime captures Qusayr, it would control all major routes between Damascus and northwestern Syria, which is a stronghold of the Alawites, the Shiite Muslim offshoot to which the Assad family belongs.

"Qusayr's importance resides in its proximity to the Lebanese border and because it is a route connecting Damascus to Syrian coastal cities, a region considered as the regime's core backbone," said Col. Aref Hammoud of the Free Syrian Army, a collection of former government forces and others fighting Assad.

Failure to hold the town would be a major setback for opposition forces, said David Hartwell, senior Middle East analyst for IHS Janes, a security consulting firm.

If that happens, "the Assad regime is likely to feel emboldened about its long-term survival prospects and can begin to turn its attention to regaining other territory — especially in the north and east — lost to what looks like an increasingly fragile opposition fighting force."

Critical to the shift in the battlefield is Hezbollah, a fighting force in southern Lebanon that has fought wars with Israel and whose members dominate the Lebanon parliament in Beirut. It is supported with arms and funds by Iran that often transit through Syria. Iran is Syria's main ally.

FSA Nour battalion Lt. Safwan, who goes by one name, told USA TODAY on the Syrian-Lebanese border that the Assad regime has left most of its Sunni brigades in its barracks and is relying on the army elite forces and on the newly founded People's Army, a 50,000-strong militia comprised mostly of fighters loyal to the regime and members of the Shiite and Alawite minorities.

The militias have been trained in guerrilla tactics suited to the urban warfare that Hezbollah fighters and Iran's Islamic Revolutionary Guard Corps are accustomed.

"These combined forces are managing operations in most areas. In Qusayr, however, Hezbollah is at forefront of the battles," says Abu Wali, another officer in the FSA Nour battalion interviewed on the border.

Retired Lebanese general and military expert Wehbe Katisha says Assad had to turn to Hezbollah to gain control of the region because the group is much more adept at guerrilla tactics than his forces.

Hezbollah had for months refused to comment on its fighters coming to the aid of Syria's government at a time when Assad is accused of killing at least 80,000 fellow Muslims in his fight to survive. But it appears that Hezbollah can no longer hide the fact.

According to the Syrian Observatory for Human Rights, 31 Hezbollah fighters were killed in Qusayr over the weekend and many funerals are being held in Lebanon for the returning dead.

Meanwhile, rebel-held areas outside the Syrian capital of Damascus are seeing more action from Hezbollah forces and militants belonging to Iraqi factions allied with al-Qaeda, Hammoud says.

Among the militias taking part in that front is the Syrian Resistance, also known as the Popular Front for the Liberation of the Sanjak of Alexandretta, headed by Mihrac Ural. Ural, a Turkish citizen and an Alawite like Assad, is accused by Syrian opposition groups abroad of being involved in a massacre at Banyas, where more than 100 civilians were killed.

FSA Lt. Abou Wali and Lt. Safwan say troops loyal to Assad are renewing offensives to control the main roads linking Damascus to the country's major cities such as Beirut and Aleppo. Rebels have established havens in towns along these highways.

The regime is also focusing on strategic areas in and around Damascus that have been taken by rebels such as Moadamiya, Daraya and Ghouta. Moadamiya and Daraya are situated close to the Mazzeh military airport; Ghouta is on the way to the Damascus international airport.

"We have been under siege for six months. The Syrian army has banned any food and medical supply from coming into town and burned down all our fields, people have nothing to eat," says Ahmad Mouadmani in a Skype conversation from Moadamiya.

Outside main cities, the regime has kept its military superiority, using airstrikes, ballistic missiles and artillery, provided by its allies Russia and Iran, say the FSA officers. The FSA, on the other hand, relies on weapons captured from regime forces and on an unsteady flow of ammunition produced locally or supplied by foreign donors.

The Obama administration has said it will soon notify Congress about an expanded package of non-lethal assistance to the Syrian rebels. But the rebels say they need lethal assistance, such as heavy weaponry to counter tanks and a Western-initiated no-fly zone, to end the slaughter.

Assad has rejected past peace overtures that would remove him from power. Kerry is pushing for just such a deal that would have Syria transition to a new government.

Meanwhile, the U.S. Senate Foreign Relations Committee voted 15-3 on Tuesday for a bill that would provide lethal assistance and military training to vetted rebel groups, and would slap sanctions on anyone — such as Iran or Russia — who sells oil or transfers arms to the Assad regime. Russia has been supplying Assad with weaponry and has approved the sale of anti-aircraft missile batteries to Syria that would blunt any attempt by Western warplanes to create a no-fly zone.

Obama has not publicly criticized Russia for the move, and Kerry is soliciting Russia's help in organizing peace talks. Russia's leader, Vladimir Putin, has said he will oppose any attempt to force Assad out militarily.

Given the lack of aid from the West, some rebels believe it may have to resort to assassinations to hold onto its positions.

"The assassination of Assef Shawkat has shown the validity of this strategic approach, one that that could intensify in the future," says Safwan, referring to the Syrian deputy minister of defense killed in a bomb attack in Damascus in July.

The conflict has proven to be highly unpredictable, however.

"The regime's control of certain areas can be a very difficult task, especially in regions where it is opposed by a large swath of the population," Katisha said.

Hartwell says the rebels may still hang on to Qusayr, where they have prepared for an offensive by building tunnels and booby-trapping homes. Should they lose control of it, the rebellion will face an uphill battle to hang onto gains elsewhere.

"Buoyed by unconditional support from Iran and the increasing nervousness of the West and Gulf states about the wisdom of beginning or continuing to arm the rebels, the Assad regime is likely to feel emboldened about its long-term survival prospects," Hartwell said.